Precoating

[marvinwus]

I am building the CS17. Would it make sense to precoat the bottom and side panels with epoxy before assembling the hull? I am using Okume BS 1088 plywood, would precoating significantly affect the stiffness of these panels?

[Kidd]

Most people coat the wood with epoxy. I did. I didn't figure out until half way through the hull just how much easier it is to coat the wood while it is laying flat. It doesn't noticibly change the stiffness of the wood to my mind.

[PeterP]

One coat of epoxy will not make your plywood any stiffer yet at the same time it will not do much to waterproof it either. You should really do at least 3 coats to waterproof (Whole lot stiffer there). And that goes better and quicker wet on wet. Also, if you plan to glass it 'takes' better on raw wood. And you save yourself the sanding cured epoxy b4 glassing. But I do agree with KIDD - spreading epoxy is most comfortable on flat surface over sawhorses. Also, some pieces have to be coated b4 you put them in the boat otherwise you'll never get to them later. PeterP

[Peter HK]

On a previous boat I had the experience of ply (the foredeck) cracking as I was fitting it, after it had been epoxied (3 coats), yet when I had dry fitted the same piece before epoxy coating it had flexed easily into position. The crack was over 12 inches long.

I'm going to leave the areas that need to flex unepoxied and do it afterwards.

Peter HK

[Doug Cameron]

In the first 24 hours, you can get a primary (chemical) bond. After that, you have to sand and only have a mechanical bond.

Also the Gugeon Bros. say that you should precoat before the thickened epoxy in the scarf or the dry wood will pull the epoxy away from the scarf and into it's spaces. Again, if you do the thickened coat soon after the pre-coat, there will be a strong chemical bond.

I would want to wait on pre-coating just to avoid the sanding (wear a good respirator -- epoxy dust is not good for your lungs).

[Charlie Jones]

On parts where it will be difficult to reach later, I pre coat, then sand, then do a second ( or third) coat, sanding that to the final smoothness. Then when the parts are installed, they are ready for paint, or can be left as is if they are not to be painted.

I have even taped off where beams land and pre painted parts, such as the underside of cabin tops, etc, before gluing in place. Prior to gluing, I tape the edges of where the glue will be to keep the epoxy off of that. Sometimes precoating and pre painting saves SO much time later that it's well worth the pains taken to keep the areas clean.

I always ALWAYS precoat plywood before I glass. I try hard to do that just before the glassing happens so I will get the chemical bond. I don't sand afterwards- I use scrapers to knock down nibs and drips. I have tried both ways and unless I absolutely have to, I always glass onto an epoxy undercoat.

Fact is, unless I have to clean up a scarf joint or something like that, I never even touch the ply with sandpaper until after the coats of epoxy are applied. If it's an area to be glassed, no sandpaper will touch that area until after glassing and all fill coats are done. Scrapers yes, sandpaper no.Then it gets a good sanding and a final thin epoxy coat applied. But to do it like that you have to work "hot" Glass, then fill in one continuous time frame, which can make for a long day or two. When I built the Princess 22, I epoxy coated the exterior of the hull, glassed the sides and bottom, and applied all fill coats in two quite long days, working completely by myself.

[JeffM]

I took Charlie's advice and epoxied all panels while flat (2-3 coats, if I remember). I had a devil of a time pulling the bow together, finding inovative ways to get more and more mechanical advantage both to draw the bottom up to the forward bulkhead, and to pull the bow in so I could wire it. I've always suspected that the epoxy made that job so difficult, but I can't say for sure because I have no other experience to compare with.

[Charlie Jones]

I doubt it Jeff. I didn't pre coat on the 20 I'm currently building and the bow sections fought me for several days before I whipped them. Getting that forward bulkhead in the right place and all snugged up was a real chore.