Skid Plate Test Materials Impact Resistance Results

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Memaquay promised me a private show in a back room of the Blue Lagoon, so I feel rewarded enough to publish the impact resistance results

I brought this down to the DIY board, where shade tree boat tinkerers and manufactured canoe skid plate installers might easier find it. The pre-test pondering and experimental set up starts here:

http://www.canoetripping.net/forums...5-the-half-arsed-skid-plate-experiment-begins

I pseudo-scientifically impact tested eight different skid plate layups. Ten if you include two done simply for color saturation that don’t really count. Some of the impact drops were too gentle, a 9oz piece of rebar dropped from 7 feet didn’t dent any of the test pieces and barely left a scuff mark.

Some were a bit extreme, a 3lb, 9oz steel Horseshoe stake, guidance dropped from 7 feet high with the pointy end down, punched deep dents in some materials and shattered others.

P3170001 by Mike McCrea, on Flickr

As expected there is a significant impact difference between kevlar felt, Dynel and Dynel with an under layer.

Test material results, subjectively observed and inspected.

TL;DR; kevlar felt is crap for impact resistance. At some foolish point I said “If I ever do another kevlar felt skid plate . . . . .” Seeing the test results I’ll never use kevlar felt again.

The assorted minor damage is hard to assess in the photos. The yellow dots on some graphite pieces were needed to denote different impact strikes.

P3170005 by Mike McCrea, on Flickr

#1, kevlar felt, graphite powder, black pigment, peel ply compression:
9 oz rebar at 6 and 7 foot drop heights, scuff mark
1lb 4oz rebar at 6 feet, minor dent
1lb 4oz rebar at 7 feet, dent and slight crack
3lb 9oz horseshoe stake at 6 feet (blunt end), large circular dent with cracks emanating
3lb 9oz horseshoe stake at 7 feet pointy end, chunks busted out cracks emanating

P3170006 by Mike McCrea, on Flickr

#2, kevlar felt, heavily epoxy saturated:
9 oz rebar at 6 and 7 foot drop height, scuff mark
1lb 4oz rebar at 6 feet, worse scuff mark
1lb 4oz rebar at 7 feet, dent with cracks
3lb 9oz horseshoe stake at 6 feet (blunt end), deep dent with cracks (we missed the center of the of the target with the first stake drop and hit the edge, busting edge pieces of kevlar felt out)
3lb 9oz horseshoe stake at 7 feet pointy end, not as close to the edge, still broke pieces out.

P3170009 by Mike McCrea, on Flickr

#3, kevlar felt, more lightly epoxy saturated:
9 oz rebar at 6 and 7 foot drop height, the usual scuff marks. We continued dropping the 9oz rebar first just to check our aim each time.
1lb 4oz rebar at 6 feet, cracks, no dent
1lb 4oz rebar at 7 feet, dent with cracks
3lb 9oz horseshoe stake at 6 feet (blunt end), dent with cracks. We again missed center target with the stake and hit an edge, busting off chunks of kevlar felt.
3lb 9oz horseshoe stake at 7 feet pointy end, no piece broken out, but a circular dent with cracks.

P3170010 by Mike McCrea, on Flickr

#4, Carbon fiber under layer, Dynel, graphite powder and black pigment, peel ply compression
9 oz rebar at 6 and 7 foot drop height. Scuff marks, barely
1lb 4oz rebar at 6 feet. Scuff mark
1lb 4oz rebar at 7 feet, one very small crack
3lb 9oz horseshoe stake at 6 feet (blunt end), shallow circular dent, no cracks emanating
3lb 9oz horseshoe stake at 7 feet pointy end, conical puncture, no cracks emanating

P3170013 by Mike McCrea, on Flickr

#5, S-glass under layer, Dynel, graphite powder and black pigment, peel ply compression
9 oz rebar at 6 and 7 foot drop height. Scuff
1lb 4oz rebar at 6 feet. Scuff
1lb 4oz rebar at 7 feet. Slight dent
3lb 9oz horseshoe stake at 6 feet (blunt end). Semi circle crack broken, no other cracks emanating
3lb 9oz horseshoe stake at 7 feet pointy end. Conical dent with X shaped cracks in the dent
(I do not know what weight S-glass I used, something lightweight)

P3170015 by Mike McCrea, on Flickr

#6, Kevlar tape under layer, Dynel, graphite powder and black pigment, peel ply compression
9 oz rebar at 6 and 7 foot drop height. Scuff
1lb 4oz rebar at 6 feet. Scuff
1lb 4oz rebar at 7 feet. Slight dent, no cracks emanating
3lb 9oz horseshoe stake at 6 feet (blunt end). Semi circle crack broken, no other cracks emanating
3lb 9oz horseshoe stake at 7 feet pointy end. Shallow conical dent with small X shaped cracks in the dent
(We dropped the horseshoe stake several times to assure we had hit the 1.5” wide kevlar tape below)

P3170017 by Mike McCrea, on Flickr

#7, E-glass under layer, Dynel, graphite powder and black pigment, peel ply compression
9 oz rebar at 6 and 7 foot drop height. Scuff
1lb 4oz rebar at 6 feet. Scuff
1lb 4oz rebar at 7 feet. Small dent
3lb 9oz horseshoe stake at 6 feet (blunt end). Semi circle crack broken, no other cracks emanating
3lb 9oz horseshoe stake at 7 feet pointy end. Conical depression, not broken through, no cracks
(It’s hard to tell apart from the epoxy pock marks, but that ancient E-glass did surprisingly well. Better than the S-glass. It was a heavier weight material than the S-glass)

P3170018 by Mike McCrea, on Flickr

#8, Single layer Dynel, graphite powder and black pigment, peel ply compression
9 oz rebar at 6 and 7 foot drop height. Scuff.
1lb 4oz rebar at 6 feet. Scuff
1lb 4oz rebar at 7 feet. Small dent.
3lb 9oz horseshoe stake at 6 feet (blunt end). Near full circle (almost broken free) crack and dent
3lb 9oz horseshoe stake at 7 feet pointy end. Conical depression, small hole dead center.

#9 and 10, Kevlar felt with red and blue color agent pigments, done simply to verify that even kevlar felt skid plates need not be as fugly. Not a valid test; I cut the peel ply covering squares too small and they remained unpeelably epoxied atop the felt, which provided an additional impact layer. On the downside I used no G/flex in the epoxy mix. None the less, both felt patches dented and cracked, staring with the 1lb 4oz bar.

P3170021 by Mike McCrea, on Flickr

P3170022 by Mike McCrea, on Flickr

For what it’s worth, the back of the vinyl siding. Hull stem thickness (and shape) in any material would not have punched through or have broken off substrate pieces as easily as thin vinyl siding. FWIW, the kevlar felts (#’s 1, 2 and 3) all punched out chunks of material and siding.

P3180024 by Mike McCrea, on Flickr

The least damaged on the backside are #4 (carbon fiber underlay), #6 (kevlar tape) and #7 (E-glass), and the E-glass fared the best of those.

The lightweight S-glass underlay (#5) fared poorly. FWIW I think that was 6oz S-glass, maybe 4oz. Single layer 5 oz Dynel (#8) also punched through on the backside.

P3180025 by Mike McCrea, on Flickr

P3180026 by Mike McCrea, on Flickr

P3180029 by Mike McCrea, on Flickr

Having completed the impact testing portion of the experiment it occurs that there is a lot I could have done better. More on those (not my future) testing improvements later.

I have sufficient undamaged skid plate areas on each piece for the next test.

That will be a largely subjective abrasion test, running some power sanders and different grits. Gloved and gowned and goggled and respiratored, making ultra-fine graphite powder and Dynel dust. Outside.

Anybody want to swing by and do that for me while I take notes and photographs?
 
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Nice work, Mike, I applaud your diligence.

Do you have a shop-vac you can connect to your sander? I don't envy you in the next step.

Alan
 
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Mike,
Great work and way to persevere! I guess I should have spoken up in your other, pre test thread. I would like to have seen a couple test samples with Vectran or nylon as an underlayer.

As for your upcoming wear testing, are you familiar with Taber tests? In my former life, I lived by those ASTM standards, particularly ASTM D4060-14. If you have access to those standards, they could provide some insight for your next series of tests.

But again, nice work.
 
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Do you have a shop-vac you can connect to your sander? I don't envy you in the next step.

I made an adaptor for the shop vac that fits my tabletop belt sanders (which would be awkward to use in the abrasion tests) and my RO sanders. I’ll try the edge of an RO first with the roughest pads I have (120), but I think it may require the big belt sander, which has no dust collection port.

I would like to have seen a couple test samples with Vectran or nylon as an underlayer.

As for your upcoming wear testing, are you familiar with Taber tests? In my former life, I lived by those ASTM standards, particularly ASTM D4060-14. If you have access to those standards, they could provide some insight for your next series of tests.

I have no Vectran or nylon fabrics in my stock. I do have a roll of 15oz or 18oz bias glass tape, which I meant to include as an underlayer but completely overlooked.

No Taber abraser in the shop either, that would be easier and more scientific than me grinding away with a belt sander.

https://www.taberindustries.com/taber-rotary-abraser

Now that the test pieces have all been laid up impact cracked there are a lot of coulda-woulda-shoulda’s improvment possibilities, but I don’t see myself laying up another batch of test fabrics and sandwiches.
 
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I’ll try the edge of an RO first with the roughest pads I have (120), but I think it may require the big belt sander, which has no dust collection port.

I think you're right. 120 grit probably won't do much more than tickle them. I'd use 60 at a minimum on the ROS and preferably rougher than that. You've got some tough stuff to chew through. Hopefully the belt sander will get job done.

This place sells good pads for cheap at 50/box if you wanted to expand your grit selection: https://www.industrialabrasives.com/

Or better yet if you can hold off a few days I'll throw some 50 and 60 grit discs in with that sail that's boxed up here waiting to get shipped back. I'll send them all out tomorrow if I can remember to grab them from home. I'll send enough so that each sample can start with a fresh disc.

Alan
 
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Some Half Arsed testing variables and oopsies

There is a lot that could be improved in that skid plate impact test. Including:

Using some composite and Royalex/T-formex sheets to better replicate canoe stems. Maybe a stripper layup and okoume stitch and glue construction style builds as well.

Using different epoxies, or different epoxy mixes heavier on the G/flex.

I could have bought a manufactured kevlar felt skid plate kit with whatever resin is included. And used every ounce of damn pricey resin in that kit, as folks are wont to do. The least expensive manufactured kits run $95. Old Towns skid plate kit is now freaking $300! I hope they are still using that uber-stinky known-to-cause-cancer-in-California polyurethane resin. (Seriously, $300 for some resin and crap kevlar felt?).

https://oldtowncanoe.johnsonoutdoor...030d974e7c0619/parts/hull-deck/skid-plate-kit

I do not know the weights of some of the underlay fabrics. The kevlar felt is Sweet Composite’s 3.5oz 4580. The Dynel is 5oz. The kevlar tape is Sweets plain weave 5oz. The S-glass is probably 6oz, maybe 4oz (it pays to label things, even scraps, I didn’t). The E-glass was thick, maybe 8oz. The carbon fiber is something from Doug, maybe he knows.

We eliminated some of the bar drops at different heights. The test squares were not big enough to drop the 3lb 9oz horseshoe stake from multiple heights. We kept dropping the 9oz bar that did nothing, just to make sure it produced scuff marks at most, but mostly to help adjust our aim for the forthcoming damage.

Accurately measuring the depth of each dent and depression is beyond my crude methodology and caliper.

I neglected to lay a strip of the (thick) bias weave (either 15 or 18oz) tape as an underlayer, that might well have been the most impact resistant. None of the underlay fabrics are laid on the bias, and that tape would have been simplest of all to install.

Each sandwich should have had a bias layer, and another test sandwich laid straight with the same materials. I bet there is an impact improvement with sandwiched threads running diagonally, but it’s too late now. I’m kicking myself for that coulda shoulda woulda omission, and if I hadn’t dismantled the bombsite and put everything away I’d be tempted to do round two (skipping the kevlar felt).

I have no realistic chance of obtaining sample weights. I could cut out the pieces and weigh them, deduct the weight of a bare vinyl siding square, times the square inches in an average sized skid plate for approximate weights. If I had an accurate enough scale (I don’t), but in any case chunks of both the test materials and the vinyl siding substrate are now missing and I plan to abrasion grind away at them next.

I do believe skid plate weight may be an important factor, especially when an ultra light weight layup boat is hundreds of dollars more than a slightly heavier layup. The resin in a typical skid plate kit weighs 1.5lbs; I’m betting that a single layer of Dynel weights an ounce or two, and a sandwiched underlayer for better impact protection weights little more.



At the least I could have washed the grime off the backside of the vinyl siding, so the dents and cracks in the substrate were more clearly visible.

Still, half-arsed is better than no-arsed at all, although I expect the crude belt-sander abrasion test will largely be a waste of time.
 
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Whoa there Mr Science, you're getting down on yourself and sounding too fussy! I have it on good authority Mr Sparkly Dangler doesn't pass an iron over his pole dancing outfit before putting on a show, so why complicate an otherwise good thing? It ain't broke. The half arsed experiment has its merits as is. I believe the sanding tests will be very helpful and to the point. I'm sure every paddler has whispered to themselves whilst looking at a skid plate purchase ( and the price) "Yeah, but do they really work?" Your tests are answering that query adequately...and amiably. You score top marks for both! I would send you recompense for all your hard work and embarrassment, but the only wad of cash I have on hand sports the image of a spry old Scotsman promising good value. Come to think of it, that sounds incredibly appropriate?! (But you can only spend it in one particular hardware store franchise.) When I'm not spending them I'm using them as bookmarks.
Please continue your investigations with safe and sound science derring do.
 
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OK, I want to see a Skid test ! Drag it on the cement, and see how far you get, before it wears through.

Maybe glass some 2 x 4's and weight them down with bricks !

Measure how far you get before they wear through !

Jim
 
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if you can hold off a few days I'll throw some 50 and 60 grit discs in with that sail that's boxed up here waiting to get shipped back. I'll send them all out tomorrow if I can remember to grab them from home. I'll send enough so that each sample can start with a fresh disc.

Alan, I’ll take you up on that. My local hardware store does not stock a great selection of RO discs, and I never use anything more coarse than 120 with the RO sanders, so I have no 50 or 60 discs in the shop.

On the plus side I can continue to procrastinate doing the abrasion tests, but I guess now I will have to do so eventually.

Looking forward to the next round, got me thinking already about my next build.

Extra tough skid plates might be handy on a 200lb motor freighter, even if the operator is a svelte farro and broccoli fanatic toting chainsaws.

FWIW I will subjectively interpret the impact testing results, starting with kevlar felt.

The kevlar felt patches, in any guise (other than inadvertently leaving the peel ply stuck atop) performed poorly. Surprisingly poorly in comparison to any of the other thinner, lighter materials; I did not think the impact differences would be as obvious.

The test kev felt heavily saturated (see many use-every-ounce DIY kit applications) fared the worst of all. The lightly saturated felt and the graphite powder with peel ply compression felt fared a little better, but are still nothing I would ever again install on a canoe.

The carbon fiber, kevlar tape and E-glass underlays had the best performance on the impact tests. The S-glass underlay was a very thin, light weight weave in comparison to the other fabrics; using different weight materials skewed the results.

No question an underlayer of more impact resistant fabric helps. In any two-layer sandwich having one piece laid on the bias would be better still. Maybe, as Stripperguy suggests, Vectran or nylon; it would at least be good to know what (commonly available) fabric yielded the best impact resistance.

The single layer of Dynel, with graphite powder and pigment under peel ply compression, was still impressive in impact resistance, not far behind the ones with cloth underlay.

I expect that the abrasion resistance with all of the Dynel covered pieces will be similar, at least until (if) I grind down into the underlayer.
 
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OK, I want to see a Skid test ! Drag it on the cement, and see how far you get, before it wears through.

Jim, I am hoping some canoe manufacturer will donate 6 new canoes (three various composite, three T-formex, please contact me for more specific hull and lay up information). I will install a different material full sized skid plate on each end and drag them around stem grinding on the nearest pavement, half off the back of the truck.

No takers? Damn, maybe if I was a famous Instagrammer or You-tuber. I already have the fast forward background music in mind.

https://www.youtube.com/watch?v=D9I4nZx8UCs
 

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I'm not sure if anyone has posted this guy's videos of building a stripper here before. He has some well put together videos. Anyway, I thought his 'impact testing' techniques would be of note here.

https://youtu.be/jrmqn6KXN3g

p.s. at 36k youtube followers I doubt anyone is giving him free canoes to shoot up

EDIT : This might be hard for some of you to watch. You have been warned.
 
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Dear Mr. Wile E. McCrea, I get that Kevlar is crap, but what is your conclusion as to which one was best for impact resistance?
 
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My son gave me a new Dewalt, sander. Hook and loop, , but not RO. It is one aggressive sander !!! I've been using it on hard woods. I but a Ceramic 40 grit disc, and it chews up wood like a beaver !

It would make a good test on your samples !

OH ! And good Luck on getting 6 Manufactorers to send you canoes !

Thanks again for the test !

Jim
 
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OH ! And good Luck on getting 6 Manufactorers to send you canoes !

Jim, that was a joking reference to BWCA’s tale of Instagrammers wanting free Merrimack canoes for the “publicity” they could provide.

I couldn’t do the sanding tests today anyway; it’s raining and that graphite powder dust is definitely outside work.
 
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Please continue your investigations with safe and sound science derring do.

Well, I am already, unexpectedly, continuing.

In yet another test panel prep oopsie I forgot to flame treat the vinyl siding before the epoxy and cloth went down. That has proven a fortuitous omission.

I was able to cleanly peel off test patches #1 (kevlar felt, graphite powder, pigment, peel plied) and #5 (S-glass, Dynel, graphite powder, pigment, peel plied). That still leaves me a couple felt and ample Dynel covered test samples to abrade away at.

P3200012 by Mike McCrea, on Flickr

P3200013 by Mike McCrea, on Flickr

Topsides of those two peeled free pieces (yellow paint pen so I can cut them both into 3” squares)

P3190002 by Mike McCrea, on Flickr

The backside of the test pads was interesting, enough that I am glad to have peeled those two pieces off. The back sides of those two pieces.

P3190006 by Mike McCrea, on Flickr

Interesting to view the impact damage on the backside. More interesting that unsaturated yellow felt (even with graphite powder & black pigment) is plainly visible on the back of #1.

I was certain that I had heavily saturated the siding before laying down the felt, and saturated the top of the felt before peel ply went on and was hand compressed. Looked good from up top through the peel ply. When in felt, wet it way the hell out. Or just never go felt.

The S-glass and Dynel sandwich was fully saturated, and I’m kinda curious about the back sides of the others.

I do not need sander abrade the peel ply layer I accidentally affixed to the red and blue kevlar felt color samples, so those could be peeled off for a look at the underside as well. Bottom sides of those two pieces:

P3190011 by Mike McCrea, on Flickr

There are some un-saturated spots in that red pigment on the bottom, and patches of acne epoxy voids on blue tinted one. Those red/blue pieces may have wetted out better than #1 because they were done with straight 105/206 epoxy, and no viscous G/flex in the mix. But, freaking kevlar felt, never ever again.
 
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Dear Mr. Wile E. McCrea, I get that Kevlar is crap, but what is your conclusion as to which one was best for impact resistance?

Subjective answers buried above in post #10.

The real answer is that I do not know. Any second layer of any fabric improved the impact resistance beyond a single layer of Dynel to varying degrees, although none by a huge (unscientific, unmeasured) amount.

Best? Maybe Stripperguy’s Vectran or nylon? Of the most commonly available materials E-glass was as successful as anything. I am (untested) certain that any of the materials I used would have provided more impact protection if laid on the bias.

(Stripperguy – Is Vectran or nylon known/shown to demonstrate better impact resistance? Is there an on-line source you would suggest?)

I am now on the horns of at least one dilemma. I trimmed those two peeled-free skid plate patches, one kevlar felt, one S-glass and Dynel, down to equal 3 inch squares of two full material layers (bless the yellow paint pen where a black Sharpie is useless), but even the heavier kevlar one weighs some too-indeterminable less than 1 ounce on my cheap shop scale. The S-glass and Dynel is lighter even by simple hand feel, and doesn’t even register on my small-weight scale

P3190003 by Mike McCrea, on Flickr

I’d love to know the comparative weights for the times X square inches of material in a typical size skid plate. I need access to one of these:

https://www.mt.com/us/en/home/produ...MIuvC7oY-R4QIVyQOGCh0BEQvrEAQYAiABEgKBA_D_BwE

I know where there are dozens, but damned if I am driving an hour down to visit the University for 30 seconds of laboratory scale weigh-in, followed by hours of long-time-no-see, can’t-leave-yet, let’s-get-lunch visitation. Drizzle, drazzle, drozzle, drone

https://www.youtube.com/watch?v=ZuvQRdDJtLA

Anyone have ideas for where to find a find access to a milligram-accurate scale to use in weighing those two pieces? Not the local meth lab thanks.

After the abrasion tests I may try to peel off what is left of the other test pieces for a look at the backsides on those as well. That peel-off was, no pun intended, revealing.

FWIW another unsurprising observation; the Dynel and S-glass patch is relatively flexible. The kevlar felt patch is very stiff, if I forced it to bend it I think it would snap. Extrapolate that to skid plate performance on different hull materials on your own.
 
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Mike,
Here is the best data I could find for Vectran

http://www.vectranfiber.com/properties/impact-resistance-and-vibration-damping/

As for the nylon, I am currently homeless, and my reference books are all in storage. But IIRC, extremely impact resistant hulls for whitewater use commonly used a couple layers of nylon, and performed better than other lamination schedules. I'll have to dig around a bit...
 
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As for the nylon
IIRC, extremely impact resistant hulls for whitewater use commonly used a couple layers of nylon, and performed better than other lamination schedules.

Glass and nylon is a long proven lamination, Wenonah’s Tuf-weave. IIRC Hellman had a bomb proof glass/nylon layup, other manufacturers as well.

Glass/nylon is heavier on a composite build, but for just skid plates maybe worth a few extra ounces of impact resistance, under Dynel for abrasion.
 
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Skid Plate Abrasion Tests

I gathered a variety of abrading tools and had at the skid plate test samples, starting with what I expected would be the least effective abrader first. (I didn’t connect the shop vac; I kinda wanted a look at the dust. Never bothered with the burr bit either)

P3280002 by Mike McCrea, on Flickr

Using the edge of an RO sander tilted at a gouging angle, approximately 15 seconds of RO pressure on a couple of the test patches, starting with 120 grit.

120 RO on the kevlar felt (Sweet Composite’s 3.5oz 4580) left a tidy pile of dust.

P3280003 by Mike McCrea, on Flickr

120 RO didn’t much begin to scuff the 5 oz Dynel, even when I ran the sander a bit longer.

P3280004 by Mike McCrea, on Flickr

Next up, 60 grit in the RO. (Thanks for the tough discs Alan. You were overgenerous; all six test pieces barely wore a single disc, even used on-edge gouging)

Kevlar felt, 15 seconds with the 60 grit in the angled RO sander cut a crescent groove into the kevlar felt.

P3280005 by Mike McCrea, on Flickr

At 15+ seconds (poorly timed) I got far enough through the Dynel with 60 grit RO, enough to build a pile of black dust.

P3280006 by Mike McCrea, on Flickr

Dust blown the weave of the underlayment (carbon fiber under Dynel in this case) to had just begun to show.

P3280008 by Mike McCrea, on Flickr

Time for the big guns and hit each test piece, grinding through into the vinyl siding substrate; a 4” belt sander, tilted front flat down with 80 grit, pressed as firmly as the old motor would allow without stalling.

Kevlar felt – Cut all the way through in 10 seconds with belt sander 80 grit

P3280009 by Mike McCrea, on Flickr

Now that I know that the belt sander and 80 grit “cuts it” I tested all of the Dynel covered patches to see how each underlayment faired under nasty abrasion.

Carbon Fiber (6.2oz twill weave) under Dynel. That piece had a bit of a head start from the RO disc, but even so took much longer to belt sand through the two layers than through the kevlar felt. Long enough that I barely cut all the carbon all the way through. In completed-test inspection that carbon fiber underlay may have been the toughest of the lot.

P3280010 by Mike McCrea, on Flickr

Kevlar tape (1140 denier yarn, 9 mils thick) under Dynel. I had such high hopes for this, but once I got through the Dynel the kevlar tape went fast. Note the fuzzywuzzies, and how easily it came off leaving the vinyl siding intact.

P3280011 by Mike McCrea, on Flickr

As with the kevlar felt I am now curious about the epoxy saturation on the bottom of that kevlar tape underlayer, and will eventually peel off all of the test patches for a look below. Kevlar be thirsty?

E-glass (probably 8 oz) under Dynel. Right up there with the carbon fiber underlay, +/-.

I had to go back for a second run with the belt sander to grind all the way through, but forgot to watch the big hand on hand Mr Clock during those belt sander runs.

P3290035 by Mike McCrea, on Flickr

I really needed a helper with a stop watch; more half-arsedness in my methodology. The questionably abraded result with the E-glass.

P3280012 by Mike McCrea, on Flickr

Single layer (5 oz) Dynel. It was, to my surprise, easier to grind all the way through with the acid-test pressure-down 80 grit belt sander than on the kevlar felt. Semi-surprise; the kevlar felt is much thicker, and largely just an unwoven vehicle to carry epoxy.

OK, OK, I’ll say it; if all you do is drag your boat up on sugar sand beaches kevlar felt will work. But for gawd’s sake at least pigment that crap.

P3280013 by Mike McCrea, on Flickr

I had already peeled off the Graphite Powder/black pigment/peel ply compressed kevlar felt and the S-glass under Dynel. If I felt ambitious I would re-epoxy them to the siding and grind away. I don’t feel ambitious, but still have some comparative torture tests in mind for all of the skid plate test patches.

One further note on the kevlar felt. Belt sanding near already impact cracked/broken areas tended to dislodge additional chunks of felt; the short compressed felt fibers had nothing to help hold them together once cracked.

Procedural note: I was goggled and respiratored during the grinding, with long sleeves and gloves. And I was still itchy-scratchy when I finished and had to go take a quick shower and change clothes. I wish I had done better at wearing PPE starting 30 years ago.
 
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