Repair Advice

Hello
Im Trevor, new to the site.

I have a friends boat i was getting ready to re- spray but on preparation found it needed a bit of repairing. Its a wooden frame (oak i think) and a ply wood skin. Its had fibreglass tape placed on the transom and the last 6 inches of the hull. To me it appears that water has gotten inbetween the glass and the ply and rotted the ply. Anyway, have stripped the back half of the boat and removed the last 12 inches of ply on the hull and the outside skin of the transom. The frame is in excellent condition.
My questions are.

1. Should i seal the frame with a wood sealer or epoxy primer before re skinning with the marine ply.

2. Should i also do the same to the marine ply before fitting.

3. Is a polyurethane sealer adhesive such as sikaflex suitable for glueing the marine ply to the hull and also use some brass screws.

Your help would be greatly appreciated.

Thanks Trevor

 

It's always a good idea to seal wood when there's a chance that water might end up sitting in the space between parts. That said, sealing wood only partially can cause problems too. In the old days, and today as well in traditional solid wood construction, on faying surfaces (surfaces between parts joined together) a bedding compound was used to fill gaps and keep water out. Today, while you still can use a bedding compound, wood is often sealed with epoxy.
When wood gets wet problems can occur with parts that have only partially been sealed with epoxy. The drying process doesn't allow for water to get out through the sealed surfaces and as a result, moisture can become trapped for periods of time inside the wood just behind the epoxy barrier, inviting mold spores. It can be worse than no epoxy in some cases. Therefore, the rule of thumb is you generally seal the entire piece or not at all. So if it's possible (and in your case it doesn't seem to be) to seal the whole of the framing, I would do it, but if you can only now epoxy seal the exposed parts of the framing, I wouldn't use epoxy on the reachable surfaces.
Definitely epoxy the inner and outer surfaces of the plywood transom. It won't rot again if you do it right. Three coats minimum. It's half likely that the oak framing is white oak anyway, which is pretty rot resistant. Did the oak have any rot? Probably not much if at all.
As far as attaching the transom, no epoxy is required (it would inadvertently seal the back of the oak) but bedding compound such as 3m 4200 or 5200 or equal would work fine. It will move with the oak, being flexible.
You'll be glassing the sides and bottom to the transom in any case, which strengthens the structure by itself.
To avoid future problems with the plywood, epoxy inside and out. I assume you'll be scarfing in the last few inches of the sides and so you have the opportunity to seal the added-on pieces inside and out (and every edge).

 

Alan has it right. I would recommend polysulfide (3M-101), instead of 3M-5200 (a polyurethane) for this application. It sticks better in wet applications. Generally, you do glue things together with these types of structures and epoxy is excellent at this, though not the only choice. Epoxy encapsulation is an option and recommended on an wood or plywood that can be fully coated (every edges, face, cutout, holes, etc.). As Alan points out, if you can't fully seal the wood, then it's best sealed with something else, like varnish, shellack, polyurethane or primer and paint. These coatings will let the wood breath, so rot is less a problem. Lastly, unless the 'glass is quite thick, it's not going to add much strength, but it will improve waterproofness, if done right. Epoxy and cloth are the only products that do this well enough to consider on wood. If it was me, I wouldn't bother with the cloth, just keep it clean, dry, repaired when you bash into stuff and painted - she'll live longer.

 

Thanks heaps guys, i will get the sealer out and start sealing.

The plywood skin is fixed with copper barbed nails and countersunk a little. They have used some sort of filler to for the countersink and its all lifted. Im going to remove all the filler, give every nail a little tap and then use the dremel to clean the head of the nail.
What should i use as a filler?
Thanks again

 

An epoxy putty would probably be best. I'll leave it to PAR to suggest a filler. Something that bonds well to metals. Roughen the area with a coarse grit of sandpaper and clean well with acetone prior to puttying.

 

heres some pics of the job
pic 1 is the repair im doing, its not as bad as it looks, structurally, the boat is great, just down the back its been rotting the plywood. i probably cut out more than i had too, but thats me, go to the good part then cut a little more to be sure.

what would you guys do with the little bit of rot in pic 2, im thinking of just getting a dremel tool in there and clean it up a bit then using some of that fibreglass reinforced body filler in there, maybe add a couple of small screws as well to give the bog something to bind to.

pic 3 is one side sanded down and ready for some primer sealer, it didnt come up too bad, few ripples here and there but im not going to spend a week blocking it back. I will hit it with a couple of coats of hi fill primer and then wet rub them back a bit and that will do.

Thanks

 

Sorry, no pictures came through. About tapping the little copper nail heads - don't. These aren't nails, but are clenches or roves. Both need to be "backed", which requires a person on the inside with a "buck" or rove set. "Re-setting" the clenches can help a lot, in a leaking boat, but more often then not, half the clenches you try to "tighten up", have the curled tip break off, which ruins the "clenching" ability of the fastener. This re-setting is careful work. Too little of pressure and you haven't done much. Too much and you'll break the clench. The person working the backing iron has to have a clue what's going on too. The process is easy, when building the boat and normally, one person, but in repairs, this isn't possible so a coordinated team must tackle the job.

I wouldn't use epoxy as filler over the roves or clenches. I'd use a rosin/beeswax mixture or a light weight acrylic filler. These will live fine under paint and are easily removed when the time comes. Epoxy will make removal very difficult without major grinding on surrounding areas.

 

Hi Par
I just had a look at clenches and roves and see what you mean, a guy is on the inside with a mandrel and bends it back over. But these things i think are nails are not like the clenches. They dont have that bend over piece, they are just like a normal flat head nails, but they have barbs along the shaft of the nail. Not a single barb, but about 10 barbs that go 360 degrees around the nail. I will take a pic of one.
But acrylic filler, you mean good old car bog, we have that in 20 litre drums lol.

Thanks again