: Many times I read on this site and other places about epoxy not
: giving any strength. Try snapping a scrap of 1/8" Luan or
: Okume and then coat a similar piece on both sides with epoxy,
: let it cure for a week and try to break it.

Jay, I'm glad you begrudge me a bit of leeway in my descriptions, but you have a point about epoxy which I tend to gloss over at times. Maybe we will still be friends after I elaborate. :)

I do not want people to get the idea that epoxy has a certain amount of strength by itself, and gless cloth has also got a certain amount of strength by itself, so (in some people's heads) if you add the two strengths you get the totoal strength. That kind of math doesn't work because the materials don't work that way.

What we are creating when we add epoxy, or polyester resin, or even hot-melt glue, to a dense collection of fibers is a "composite" material which is also refered to as FRP, or Fiber Reinforced Plastic. It is a beautifully descriptive name. If people would keep that term in mind they may gain a bit more understanding of the process of how composites work.

Lets look at something similar first: concrete. We heat limestone until we break down the chemical bonds that hold the water in its molecules, and the left over powder is our cement. Add some water back to it, and give it a week or more of moisture, and we can cast blocks of a new stone from this cement alone. We have found that these new blocks are fairly soft, but that we can mix in some tougher rocks, like granite, sand from quartz, or even more limestone, and get a stronger result. If we use the cement to glue loose rocks together it strectes our supply of cement, so you might add 4 parts of sand or rocks to one part of cement. Add too many rocks and the cement is too widely dispersed to hold them together. Add too few rocks and your mix is just the unenhaced cement. Somewhere in the middle of this range we create finer and coarser concretes for paving roads, casting foundations, gluing together bricks (as mortar) and filling fencepost holes. Oh yeah, they make boat hulls from the stuff, too.

I'm in Indiana where they have a lot of quarries from which they rip limestone on a daily basis. It is a fairly easy task which primitive people learned to do long before metals were discovered. Use a hard rock to scratch a hole in a softer rock, jam in a stick, soak the stick for a week, and as the extra water makes the wood fibers swell, it expands and splits the rock. Trees sending roots into cracks do this naturally. Water dripping into cracks , and then freezing does it too. Most rocks have great strength when they are resisting a force which tries to compress them, but they have much less resistance to being pulled apart.

It is road repair season by my house. The crews are pouring fresh concrete over loosely laid grids of reinforcing iron (rebar). The size of the grid openings is important. The rebar will stretch if you pull it hard enough. but it is pretty ductile, so it will strecth a lot before it will break. If it was good and hot you could strecth a block of steel into a thin wire a mile long. When it is at room temperature it won't stretch anywhere near as far, but it is not brittle, and it does have a strength we can test for and measure.

The magic happens when we add that stretchable rebar to a mix of a weak glue and crushed rocks. One material stabilizes the other, and the result, reinforced concrete, has better properties than either material alone. The hardened cement, prevents any motion of the rocks in the matrix. Those trapped rocks fill the metal grid, and reduce the ability of the rebar to stretch. When the concrete is subjected to compression the rebar adds a little strength, bt it is already very strong in that area, so the improvement is not significant. But when the concrete is sibjected to some forces that want to stretch it, like expansion from heat on a hot summer's day, The rebar constrains the stretching. The area in the middle is pushing against the ends (which are being pulled together by the rebar). This action actually can change the direction of the forces involved, and convert them into a direction where they are under compression --an action they are very strong at resisting.

Lets consider your plywood examples. If you bet a chance, by the way, try the same experiments with polyester resin, or maybe we can get Sam McFadden to devise some fun tests for comparing these materials. As i se it, the epoxy resin will soak deeply into the fibers of the wood. Working with 1/8th inch plywood and warm resin it is possible for the resin on either side to soak in 1/8th inch, which is way past the middle of the panel, so lets assume it goes in 1/16th of an inch from each side and the panel is totally saturated.

When make paper we tear apart wood by heating it and beating it. the heat and agitation destroy the lignin which binds the celluose fibers together. Wash out that lignin glue and you have clean cellulose fibers, which are obviously stronger than the glue lignin, because they survived the whole process and the lignin didn't. The people at West Systems tell us that their epoxy is about 12 times stronger than wood. I'd guess that epoxy is probably in the range of being 12 times better as an adhesive than the natural lignin produced in the tree, and when epoxy saturates wood, it makes a bond between those cellulose fibers which is 12 times stronger than nature.

Once you coat your plywood with epoxy you do not have merely a surface coating. You have transformed wood into a fiber reinforce plastic with a thickness of about 1/16" to 1/8th", depending on how well the epoxy soaks in before it hardens. This is going to happen with a saturating precoat, or with a wetout coat if the wood is not presealed.

So, the epoxy in the wood turns that into a thick FRP. The glass embedded in the wetout coat is another layer of FRP. Same plastic (the epoxy) but a different kind of fiber reinforcement. Glass fibers here instead of wood fibers.

Then what goes on top of that? A layer of unreinforced epoxy. In this case it is a very thin layer--about as thin as a coat of paint, or maybe thinner. The resin which soaks into the wood is thick, but it is hard to determine the exact thickness without cutting into the boat. The resin in the cloth we can estimate at being slightly thicker than the cloth itself. If I remember correctly, 4 ounce cloth is about 8/1000" thick, so if the resin is applied with a squeegee and is just thick enough to barely cover, the result will be about 10/1000", or about 1/100" thick. 6 ounce cloth would be about 50% thicker. Since the plastic in all these layers is reinforced, we have a composite material here. Wehn we use a squeegee to apply the resin the thickness of the fabric serves as thickness gauge to control the layer of resin. Anything thinner than the full thickness of the fabric and the fabric won't wet out. Anything thicker is scraped off by the edge of the squeegee as it rides along on the high spots of the fabric weave. The epoxy is very "runny" and if too much is applied it will drip down on vertical surfaces before it hardens. the capillary action of hte fabric keeps that resin in place. Otherwise, the coating would probably be under 2/1000" thick. So, having fabric there makes the coating more than 5 times thicker than it would otherwise be.

If one were to use an unwoven fiberglass mat the layer would be much thicker, as the mat would swell and soak up resin like a greedy sponge. But the percentage of glass would be lower, and we know that unwoven mat/resin mixes are not as strong (for their weight) as woven cloth/resin mixes.

If the "perfect" strength to weight balance between epoxy and glass is a mix of roughly 50/50 by weight, a squeegee applied coating will typically be a bit rich in resin. Probably something like 60% resin to 40% glass cloth. Certainly strong enough, but with the excess resin it will be a bit heavier than it might be. I think to get a really lean mix of resin one would probably need to use vacuum bagging.

After this point, any additional resin which is applied is a thinner coat, about 2/1000, because otherwise it will run. Even if we put on three coats, we will probably sand off a lot of the material in these added layers sa we try to remove the texture from the weave. The tendency of the resin to follow the curvature of the underlying texture, or "telegraph" the weave pattern, leads first-time builders to put on one or two coats too many before they sand off the excess to achieve a smooth surface. Then they add varnish or urethane on top to protect the epoxy from scratches and UV light damage.

The weight slowly adds up, but after the wetout coat and the application of the glass, all the subsequent coatings are unreinforced or unfilled. It is resin alone, or varnish, or paint. There is no synergy with an embedded fiber to give these later coats strength on the order of the fiberglass under them.

: Take 2 pieces of plywood and coat it with a layer of glass with
: epoxy. Fill the weave on the 2nd piece. Then put a cement block
: on it and drag it down the street with a rope and see what
: happens. The un filled glass one will be shredded apart.

Why such a wimpy experiment? Forget the cinder blocks, put a 120 pound teenager on a skateboard and let him "grind" it along street curbs. :) You'll prove your point either way: A top coating does protect against damage from abrasion. But the question I'd like to explore here is: Which coating does the best job for the lowest cost and lightest weight?

So, lets keep either of these this experiments going for a few miles more. and add on a third panel of plain, uncoated plywood. Once the glass fabric starts to be snagged it gets ripped out of the bed of resin it is in. That certainly looks bad, but the panel which has a second coat will eventually get shredded once that topcoat is scratched through. The good part of this experiment should be yet to come. ideally there is enough resin soaked into the wood that you can drag those boards several miles before the wood fails. The uncoated piece will not be so lucky. It will be gone a long ways back. The moral here would be that either of these uses of epoxy and glass do a great job of reinforcing the wood we build our boats from. One is just a bit better than the other. And that is why we add another coat of resin over the wetout coat.

But why add expensive epoxy for fill coat? You could get a similar effect with any coating; although once it is completely cured, epoxy is probably better than almost all of them for this kind of protection. Polyester resin might be stronger for this, though. The amount of protection would vary with the material and the thickness. 5 layers of house paint would last longer than one layer, three layers of epoxy would last longer than one of epoxy. A tougher finish would last longer than a softer finish. That might be a good reason to try using something other than epoxy for fill coats.

The exact nature of the coating would give you more or less scratch resistance. I suspect a urethane coating such as is used on automobiles or floor surfaces would be tougher than epoxy. I'm not sure how polyester resin would rank with urethane, but I have heard of it being used by some surfboard makers as a top coat over epoxy. I've also read that it might not bond very well to epoxy, so it might not be the ideal topcoat! Urethanes bond well to epoxies, and they are a little less expensive. In common practice we fill the weave with epoxy, and then many poeple use a urethane based finish as a varnish. Maybe we would get better scratch resistance if we filled the weave with a clear urethane instead of epoxy! If there is a good brand of urethane or polyurethane which would self level and fill the weave on a fill coat without extra sanding it would be perfect. Urethanes dry faster than epoxy cures, so we could get the boat out sooner. Even if we had to put on four layers, they could all go on in one day. Self leveling would reduce the amount of sanding, saving time, costs, and unhealthy airborne dust. Now all we need to do is find such a miracle finsih.

: We work a balance with strength / weight by using a binder and
: glass fabric in combination. But don't under-estimate the
: abrasion resistance that the epoxy supplies. I know when I
: repair a boat, the sander is slowly grinding away as it tries to
: work through the hardened epoxy and once I hit the glass, it
: easily cuts through it. The glass is a girdle holding the boat
: together and becomes a puncture proof fabric because its fibers
: are held in place.

Excellent points and I agree with you completely on this.

: But epoxy itself is a super strong chemical
: and many applications use it alone to make castings that you can
: actually tap for a bolt or as a stand alone part quite able of
: taking all kinds of stress.

Time for a hockey metaphor. When we let excess mixed resin harden in the bottom of the mixing cup it turns into a hard, heavy blob that vaguely resembles a hockey puck. As most hockey players can affirm--getting hit in the mouth with a hockey puck can loosen your teeth. If it hasn't happened to them, it has happened to friends.But that is an inch-thick puck. That is quite a contrast to being hit in the face with a piece of paper. A paint-thin layer of epoxy or urethane*thinner than a piece of paper) sitting on top of the structural FRP layer is going to add marginal strength, if any, to the boat. put it on thicker you get more ding resistance. Put it on as thick as a hockey puck and the boat might as well be armor plated. It will crtainly be stronger, but it is going to be much heavier.

So I stand by my assertion: Without added reinforcement those added layers of resin don't add strength. But I'll also quibble: If they do add any strength it is marginal. Adding too much unreinforced resin might give you an increase in strength but it would be at a great cost for weight and materials. For more stregth add more glass with resin, not resin alone.

: Paul, Im not jumping on you, - you gave a lot of good advice. I
: just want to clarify a misunderstanding about the strength epoxy
: gives to our boats.

Go ahead and jump. You are far enough away that it can't hurt me :) Glad you brought up the topic, and hope you don't mind my long analogies. They are meant for others who read these posts. Discussion beings enlightenment, and maybe someone will play with urethanes for fill coats and, with a stroke of metaphorical lightning, bring lightening as well as enlightenment.

PGJ