Thread: To sheath or not to sheath. Repair plywood checking

It's hard to tell from the photos, but the general rule is, if the boat is plank (plywood) over frames, then don't sheath. Sheathing the outside from rail to rail can improve abrasion resistance considerably, particularly if you use Xynole. This is just a epoxy and fabric coating that protects the surface of the wood and offers no structural stiffness.

On the inside, you generally want to leave things painted or varnished, if plank over frames. Again in areas where you expect some heavy wear or foot traffic, some might use sections of fabric, but personally, I wouldn't and will just employ well maintained paint, which is a whole lot easier to touch up, repair and renew then patches of sheathings between stringers and ribs.

Checks are a fact of life with some species. Grind them out a bit and fill with thickened epoxy. Under paint, this is invisible. If under varnish, then just fill the checks with epoxy and wood flour with as close a color match as you can get.

Thanks for that. I might need mention this is a novice racing dinghy and the hull is 4mm ply. If I had to grind out any checking I fear there wood be no boat left.

You mentioned the pictures were unclear I have hundreds so if there is anything I could post that could assist with your expert guidance please ask.

Thanks again.
Terry

Another picture of the boat.

Howdy,

I agree with PAR for the most part.

However I have found that once checking starts it is hard to stop. The epoxy certainly helps.

A way I fixed up an old sabre a few years ago was to mark the checked areas just outside the boundaries of the damage with dark pencil.

Then heavily sand the checked areas - but at the same time try really hard not to go through the top veneer.

This makes a really shallow hollow.

If you use regular glass (4oz or 6oz) you need quite a deep trench and it adds a fair bit of weight. But if using a very light glass ( less than 2oz - less than 75gsm) it hides very well in even a very shallow sanding depression.

You must use epoxy as it sticks to wood properly.

For Painted areas

After the glass is cured the surface can be filled over the glass with epoxy mixed with a low strength sanding filler powder. This is put on not too thickly and then sanded using a torture board.

For Varnished area

if you make the sanded area a bit shallower then you can do the glassing but fill the cloth weave with repeated coats of epoxy (use the wet on wet method) and then sand and varnish over the top.

for info on glassing, repair, wet on wet see the resources on my FAQ here.
FAQ - Boat Building and Repair Methods - Plywood Epoxy Fibreglass Cedar Strip - Michael Storer Wooden Boat Plans

Generally it is not worth glassing the whole surface. Like Paul (PAR) indicates it is a lot of trouble and adds a lot of weight. But choosing areas to repair and using light glass makes sense - I think, anyhow

I don't have a problem with a light cloth on the checks either. Some species, like Douglas fir need a fairly heavy sheath to control checking, but most species will live nicely with just a wood flour and epoxy filler, which saves, weight, some costs and effort. The "dishing" technique that Mik mentions is one I use. It's fast and easy with a little practice. I treat most checks as if it's any type of damage in for repair. Grind (shallow depressions or worse as required), fill, fair and paint. Epoxy is called the wood butcher's friend for this very reason.

Thanks for responding I will go the patch approach and use cloth where I don't feel I can safely grind out the checking.

Cheers
Terry

PS: Anyone know where I can get lightweight tape in melbourne...preferably less than 150gsm (apologies for the sub thread)

I wouldn't use tape on the repairs, but cut patches from regular cloth, so you don't have to deal with the tape edges. Also place the cloth on a bias, so the 90 degree cross hatch pattern of the fabric is an "X" over the longitudinal axis of the check. This places the 'glass fibers in their best orientation to control further checking.

Originally Posted by PAR

I wouldn't use tape on the repairs, but cut patches from regular cloth, so you don't have to deal with the tape edges. Also place the cloth on a bias, so the 90 degree cross hatch pattern of the fabric is an "X" over the longitudinal axis of the check. This places the 'glass fibers in their best orientation to control further checking.

Excellent I will do exactly that, maybe take some photos of the process to help the next noob who comes along.

Thanks gusy.

Unless you suspect the wood species is prone to checking, I wouldn't go crazy with the cloth. filling with thickened epoxy will fix the checks, again unless you think the species needs additional reinforcement.

Hi Terry,
I have been fixing some checked ply similar to yours. I followed the excellent advice from this group and ended up filling the cracks with epoxy & lightweight filler then sheathing in the lightest glass cloth I could find 85gm/sqm.

I am very happy with the result, the light glass is very easy to drape and to make sure I did not use too much resin I weighed the glass and used a 60/40 epoxy/glass ratio to estimate the weight of resin to use. The glass is saturated but still a little bumpy, the remaining weave fills easily with undercoat.

I got the glass, epoxy and a bunch of other stuff from Phillipe at Build Wooden Boats in Preston. Nice guy, very friendly team there and some interesting boats being built in their shop.

I will take some photos of what it looks like tomorrow.

Regards,
David

Duckflat has 2oz (75gsm cloth). They could post some to you - it doesn't weigh much. Also you could see if you have an FGI outlet near you.

Phillipe is a good chap too.

MIK

To avoid getting bumps in the glass surface for the first coat wet the plywood area with either a brush or roller depending how big the area is. Then drape the glass over the wetted area and use yr hand to smooth out any bumps in the glass. The idea is you just want enough resin to hold the glass down.
Once the glass is laying flat over the surface then use a roller or brush to totally saturate the glass.
Then i use a surfboard makers piece of rubber to force out all the excess resin and this will ensure the glass is as close the the plywood surface as possible.
You will find the surface will be very flat with no resin buildup under the cloth.
Let the resin go off and apply a second coat to fill the cloth depending on glass weight you are using it may take 2 or 3 coats to fill. Then sand till you are happy the finish.

Hi guys,
Here are some poor quality pictures of the sheathed checking on my boat.