Monday, February 28, 2011
Epoxy/Fillet/Cloth-Tape Details
Itamar, Dan #185 left a comment on my last post requesting more detail on why gluing both hull side top hull bottom joints and the transom in one go was a mistake. First let me explain I'm new to the blog thing and don't know how to reply solely to him, so I'll just post what I know. Also, although I've wanted to build a boat for years this is my first build so I'm still on a learning curve too. Here I go, I've read the West System User Manual several times plus I know a very experienced builder who is always willing to answers my questions. The manual is free at any West System supplier and available on the net at their web-site. After studying this manual two principles keep going through my mind: 1) a chemical bond between layers of epoxy is much better than a mechanical bond , and 2) clean-up the blush before applying another coat of epoxy. With these two thoughts in mind I plan my glue jobs such that I will apply a new coat of epoxy to 'already cured' layers of epoxy as few times as possible over the course of build. This assures I achieve the chemical bond and avoid the blush. These are the steps I take to glue a joint: 1) prep the joint, sand the wood with 80 grit and brush/vacuum to remove all dust, 2) wet out the joint with only mixed epoxy, let it soak into the wood, 65-70 degrees F with slow hardener allows the glue to really soak in. 3) once the wet coat tacks-up apply the fillet, that's another discussion, 4) once, the fillet tacks-up apply another wet coat of epoxy, without delay neatly lay the cloth into the wet epoxy, 5) apply additional epoxy where the cloth looks dry, I especially concentrate on the filleted area where most of the stress is on the joint, 6) let the three layers of epoxy tack-up, and apply one more layer of epoxy to fill the weave of the cloth, 7) now, I place release fabric over the fresh coat of epoxy to minimize blush and prevent having to sand any cloth fabric fibers, 8) when all exopy is cured remove the release fabric, you'll be pulling any blush off with the release fabric. When we glued the hull's joints in one go temperature was alittle over 50 degrees so the epoxy wasn't as liquidy as I like so I had to message it into the wood, and cloth, Time Consuming! I also used fast hardener, more pressure to move fast! On top of that your just about standing on your head, all-day! It was an accomplishment but I'll plan differently next time. It's well worth the effort and expense to practice this on a few scrap pieces of plywood before doing it to the boat. Then break this practice piece and see if the wood is failing and not the glue joint. If the wood is failing you must be doing it correctly. It's all FUN!
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Chemical versus mechanical bond, not "much" better, just a little better if the mechanical surface is well prepared- try a test piece or two.
ReplyDeleteAlso, I'd suggest wetting the cloth on a plastic covered table and then laying it onto your wetted wood and fillet. I like to use fast resin for the wood wet out so it gets tacky quickly, slow resin for the fillet so it doesn't auto-cook (more important with big fillets like the acute angles, and hot days), and I usually don't bother to wait for the fillet to firm up- tooling the tape into the fillet with a double-gloved finger is a nice way to get smooth results, and you can push the stuff around at the ends to help bond the tricky parts.
This way, you can tape a single joint in about a half hour if that is the time chunk you have available, instead of being committed to a whole day process of consecutive cure times. I'm not a rockstar glass guy, but these are the things that have worked better for me.