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Hello all,

I've just gotten back from a boat building adventure and have a couple questions about epoxy joints.

The first concerns taped seams.  Which is better, laying fiberglass cloth over the glue joint and then filleting over that, or filleting the glue joint first then glassing over the fillet?  My first instinct is to glass first, then fillet.  Right? Wrong?

My second question is which is stronger, a wet on green or wet on cured and sanded? 

These questions seem to have a direct effect on the speed and simplicity of accomplishing these joints.  My next build is likely to be a glued lapstrake build so this may become inportant to me this summer.

Al

To me the best person to answer your second question would be Paul but my understanding is that adding fesh to still green makes for one bond as the new chemically bonds with  the green during a given window ot time.  When I did layers of epoxy, on my boat hull and the floor, I set my alarm and went out every 4 hours to add a new coat.  It was time consuming and I really had to beat myself out of bed for the midnight and 4 am coats but I wanted them to bond without having to sand between coats.  I over did it of course.  I don't think it really needed 5 coats of epoxy with a layer of cloth included but I was new and had not read Paul's post about using a light filler to make epoxy bondo and was filling weave with at least two coats that could have been reduced to one with a light filler added.

The way taped seams work is you fillet the joint first, which makes a radius at the intersection, so the fabric will lay down nice, then you apply tape. Not all joints are taped BTW.

The two bonds as Terry describes are: chemical and mechanical. A mechanical bond is when you have a dry, previously epoxied substrate and you're bonding to it. This is a mechanical "keying" process, where the surface is sufficiently roughed up, so the goo has something to grip. This isn't as strong as a chemical bond. A chemical bond is when the previous epoxy coating is still chemically active, so when fresh goo goes over it, the molecules physically interact and bond to each other. This is stronger, but it's nearly imposable to build a boat without some mechanical bonds, so not as big a concern (if done properly) as it seems.

Epoxy bonds fail mostly from prep. If the surface isn't rough enough, you have contamination or the mix ratio isn't right (etc.) the bond line will fail. If the prep is right, mechanical bonds on wood, will usually be stronger then the wood fibers can tolerate under load and a failure will occur in the fiber, not the bond line.

Glued lapstrake builds have very little 'glass work, mostly because you can't sheath lapped joints effectively, by hand, without ruining the laps with big fillets, to make the joint smooth enough for the fabric to lay down. This is why you apply the fillet first, as 'glass fabrics don't like to make hard turns. The same is true of a chine or corner. You have to round over these edges, significantly to get the fabric to lay down without puckers, folds, wrinkles or voids. The best demonstration is to just mix up a small batch of goo and try to apply cloth to an inside and an outside corner, without having a fillet or having rounded the outside corner over.

On an inside corner, without a fillet, you'll get a void under the 'glass, which will just break or shear clean under load. On an outside corner (no radius) the fabric will pucker, wrinkle, etc. and the same thing happens: you don't have full contact with the surface and the joint sucks.

One of the fastest and certainly the lightest wooden build methods, is glued lapstrake. The reason is, you can shape the planks on a work bench and you don't usually sheath the boat, both saving time and materials. It's common to sheath the garboards on a lapped hull, but the rest of the planks are often not, just epoxy coated. I have seen some builders sheath the planks separately, while still on the work bench. This is done carefully, so the lap faying surfaces are still wood to wood contact and the exposed edges, the most vulnerable part of a lap plank, are still unprotected (can't wrap a hard edge thing), so the sheathing process is dubious at best, without considerable care and effort.

Thanks Paul,

That answered my question perfectly.  Yes, on the  boats we were  building there was both lapstrake and chine joints.  On one of the  boats, I sanded and filleted the chine joint after the glue had set, then before the fillet had gotten very hard, I poked the glass down on it and let it wet out from the fillet goo.  Right after that I painted on a nice even coat of wet out epoxy to cover the whole thing.  I figured that this way I had a chemical bond on everything above the glue joint. 

Al

You got it right Al.



This is a section of a drawing I send with every set of plans, which details how to scale a fillet for it's task, based on material thickness. If you email me (click on my icon) I can send you the full version.