After the first fill coat on the hull we’ve noticed a number of runs on the side panel—I’m guessing we just applied to bit too much epoxy (we used a rubber squeegee—repurposed window squeegee). We’ll be more careful next time and can also go over it again with a foam brush to smooth it out. But would it be better to let it cure and then sand it now before applying the next fill coat vs applying the fill coat now (within 12 hrs) and sanding later? Is there a tangible advantage to having a chemical vs mechanical bond for subsequent coats of epoxy? For the sport sectional, in contrast it seems to other designs, there is a lot to do after fiberglassing the deck and hull and so the fill coats are going on after the initial coat of epoxy has cured. So I wasn’t sure if the fill coats are best applied during the window where they will chemically bond to each other. And even if so, I’m still wondering if it is best to smooth out the runs now anyway.
Epoxy bonds are pretty much all chemical in nature. Even the so-called mechanical bonds are the result of resin and hardener cross-linking. For materials that are compatible with epoxy adhesion, like wood, the epoxy forms covalent bonds with the wood itself. That’s a chemical, not mechanical bond.
For materials that are inert and do not form covalent bonds, such as polyethylene, roughening the surface forms microscopic pores that fill with epoxy which then cross-links with itself and anchors itself to the material. If the substrate is weaker than the epoxy, pulling on the epoxy will break the anchors free and the epoxy will peel off.
When epoxy is put onto a previous coat, if the previous coat is not fully cured there will be unreacted material available to cross-link with the new coat. When applied over fully cured coats, there will still be the covalent bonds as with wood, so there will still be a chemical bond, just somewhat weaker than the cross-linking.
Sanding the previous coat makes the bond stronger not so much because the new epoxy forms anchors (though there’s probably some of that going on), but because at a microscopic scale, no matter how carefully the resin and hardener were mixed, there are pockets of unreacted hardener and resin. Sanding exposes these pockets and gives the new epoxy something to cross-link to. At the same time, the new epoxy forms covalent bonds with the existing cured epoxy. That’s 3 kinds of bonding going on at once.
So the take-away is that whether you wait for the previous coat to fully cure or not, you will get a good solid bond, certainly strong enough for the type of boats that we build here.
With that in mind, my preference would be to smooth the surface as much as is feasible without sanding into the glass before applying the fill coat. I just find it easier to apply thin even coats that don’t run when I’m starting with a smoother surface. Otherwise I’m just reinforcing the hills and increasing the amount of sanding that I have waiting for me.
Laszlo
Thanks Laszlo, that makes sense. If by this afternoon we are 24 hrs from last coat (temp is 69 degrees in basement) is it safe to sand, or wait till tomorrow?
Curing is too time and temperature dependent and basements are not labs, so rather than trying to calculate the result I suggest that you test it. Here’s how I check my work to determine it’s actual vs predicted state:
- Touch the surface with a gloved finger.
- If it’s wet, wait.
- If it’s dry but tacky, it’s ready for another coat.
- Press the dry, non-tacky surface with a thumbnail
- If it indents, wait before sanding
- If it doesn’t indent, try sanding (just a few swipes)
- If the paper gums up, wait
- If little balls of epoxy come off, wait
- If the paper stays clean and sanding makes fine dry dust, it’s ready to sand.
Note that a full cure can take weeks unless you get the epoxy above its glass transition temperature (typically 122 to 140 degrees Fahrenheit). You can do this by putting the fully-cured workpiece into a black plastic garbage bag and putting it out into the summer afternoon sun for a few hours. This is a rough and ready form of post-curing. It finishes up any unfinished curing and eliminates most of the pockets of unreacted components, significantly improving the strength of the object. I’ve used it when making carbon fiber masts. For those I had to tape several bags together. Conceivably you could do this to a whole kayak.
Laszlo