I had this problem with a skim coat of vinylester, trying to fair the top edge of my transom core to the existing cap. Either Chris Craft did a weird job at this transition originally, or there was a funny repair done sometime in the past. The vinylester is from Merritt Supply and mixed at about 1%-1.5%, and thickened to peanut butter consistency with Total Boat silica.

I made another batch similarly and added some chop strand glass when I filleted the hull corners with a 3/4” radius. A lot of these corners were weird from the factory, so I wanted to strengthen them up as best I could.

In both cases I first brushed the surface with a bit of the un-thickened resin as a primer, of sorts.

What would cause the cracking like this?
And why are all my fillets just fine, but mixed the same (but for the glass fibers)?

Thanks for anyone’s help.

Im not sure in your case but ive had that happen to me when i its put on too thick and the resin kicks off early and generate too much heat. Maybe the "primer" layer started kicking off and generating heat and when you put the top layer on it caused a thermal reaction the rest couldn't keep up with??? Or it just too thick in those sections and again the heat cracked it during curing??
Is it also possible you didn't properly mixt it up right? maybe pockets of more hardener then the rest of the mix? I usually use a two cup method...mix the resin and hardener in one cup then pour it out into another cup and mix it again just to be sure i ddint have any unmixed resin sitting in the corners or on the side walls of the cup...

Thanks for your input, Wally! Much appreciated!

I suppose anything is possible, but I’m pretty diligent about stirring off the sides and bottom of the cup and also intermittently stirring from bottom to top. I do this for a full 2 minutes. Plus it got a bit more stirring in the thickener. The issue is consistent across the entire top edge of the transom, so I don’t *think* it’s an inconsistent mix. I do like the idea of using two cups though, just as insurance.
This will use cups at twice the rate, do you have a method of reusing them or is it just an expense of the job?

Also, I can see the possibility of the primer layer kicking off just as I was layering on the PB. I’m a total newbie at this, so I’m not 100% sure what “too thick” is for this product, but the inside of all my fillets are certainly thicker than any part of this skim coat.

Now, I *was* planning on using this same mixture as the bedding compound for my stringers and bulkheads, but I’m just a bit concerned about that now.
I still have about 3 gallons of Core-Bond I can use for bedding, but I liked the idea of the vinylester PB because any “squeeze-out” I could just use to start forming the fillets into the hull.

By the way Wally, I see you’re in Chicago, I’m out by the Fox Lake area.
Where do you get your resin? Shipping and haz-mat is a bear…

My wife and i go through about 3 containers of Greek yogurt a week from Costco and the plastic they are made from the resin don't stick...so i can mix...pour into another cup mix again and use it...then they both cure and i can just peel the cured resin right out and reuse it its good for about 5-6 pours then the cups get kinda flimsy from the heat depending on how much is left over inside it....i have a stash of about 15-20 container at a time in the garage...even the lids are good for mixing small batches of the PB and it will peel off too

The projects i've been doing as of late never really use more then a gallon or so of resin so i just get it anywhere i can at the time...ive been getting it from a place local to work called Finnishmaster. Ive even got a body/paint shop behind where i work and have asked him to get me some stuff in the past as well...works out for both of us if he needs to hit a certain $ amount for an order too
But yeah resins have gone CRAZY in the past few years...we use an epoxy resin at work for potting some of the pumps we make....price of a 55gal drum of epoxy we use went from like $600 for a barrel to over $1100 now!

You are resin rich on the silica mixture and not on the milled fiber mixture. Resin is very brittle and can shrink, causing cracks when resin rich. You have much more material in the mixture that has the milled fiber, that's why it's not cracking. We always mix 70% milled fiber and 30% silica and have never had issue and it goes on much smoother than just milled fiber.

Hey, thanks Washed Ashore!!

Obviously since being such a noob, I’m learning as I go here. Luckily it was in a spot no so critical and easy to fix.

When you say 70% milled fiber and 30% silica, I’m assuming you’re talking by weight? That silica has a lot of volume and not so much weight.
And when you say resin rich, do you mean there’s too much vinylester to mekp? Right now for a 20 oz cup, I add about 8cc MEKP, or a little over 1%. The bucket says to use 1-2%, so should I aim for a little more MEKP?

Would you use this 70-30 additive as bedding for stringers and bulkheads?

Thanks again!

Stinger,

Home | WEST SYSTEM Epoxy

Why anybody still uses PE resin is beyond me.

Thanks. Brad.

I've always used U.S. Composites. High quality and never a problem. Polyester Resins Also I mixed too much so I poured it in a cup, about 2" deep, hardened beautifully.

No problem,
70-30 by volume and resin rich is to much resin to material, on the catalyst 1-1.5 is perfect. You can bed the stringers in with it but my shop has found that leaving about 1/4" gap around the bulkhead keeps it from beating its way through the boat (stress cracks)