I recently noticed that the last two boards I made have a bunch of stress cracks on the bottom perpendicular to the stringer. I searched the archives and found a couple posts about this and the general consensus is that the cracks are fairly common and shouldn’t cause much concern if the board is still water tight. However, I notice that this only happens on the boards I glass as opposed to my purchased boards. Does anyone know how the pro glassers prevent this from happening?
Both boards are glassed with a single layer of 4oz on the bottom and 4+6oz on the deck (no gloss coat). And the stress cracks started happening after only one or two surfs. I like to keep my boards light, but do you think a layer of 6oz on the bottom might solve this? Or is the problem in the hotcoat? Any help is appreciated.
Could you elaborate on this?..I’m not sure what you mean. I use UV resin for both the lam and the hotcoat. As far as I can tell, the cracks are only in the hotcoat.
I had the same problem. But after reading lots of posts on Swaylocks and doing some experimenting, I came up with the reason and a solution.
The reason: My hotcoats were too thick. Resin without reinforcement (fiberglass cloth or carbon fiber cloth etc.) is NOT strong under flexing conditions. It just flat-out breaks.
The solution: I started sanding down my hotcoats so I could see the weave just starting to show, then applied a thin gloss. After I did it that way, there were no more stress cracks at all. If you aren’t going to gloss, you have to tread that fine line between not sanding enough (resin too thick) -and sanding too much (sanding into the cloth).
Also, I’ve read that a certain foam manufacturer claimed that his experiments showed that the flex of a board was mostly determined by the stiffness of the deck since delaminations show first on the deck. I don’t know if I agree with that 100%, but anything that stiffens the board will help with too much flex.
Every board will have some flex. The trick is to make sure that flex doesn’t cause cracking of thick, brittle resin.
Doug, that sounds like it could be the problem. Next time, I’ll try sanding more. I’m nervous that I’ll sand too much into the weave, but I guess I can always hide that with a thin layer of gloss coat. Why wouldn’t the gloss coat have the same problem as the hot coat?
Both boards I’m referring to are fairly thin shortboards between 2.25" and 2.375" thick. Below are pictures of each. The first is a 5-10 quad and the second is a 6-4 round pin.
Balsa, by the way, if you look closely at tail of the quad you can see one of the clark foam lams that you sent me a while back…many thanks for those!
I agree with Balsa on that. The hotcoat is designed to go off fairly quickly so there’s little run-off of the resin, and you get a thick enough layer of resin to fine tune the board, especially the edges. Gloss, on the other hand, is supposed to gel slower so it settles down and flows into an even, smooth surface. If it does that, and the room is clean with no dust in the gloss, the finishing is done with very fine sandpaper and polishing compounds. But it’s very thin: designed to fill scratches and give a nice polish.