Protecting exposed plywood edges

[jbo_c]

Working on the roof of my boat and wondering how to best protect the plywood edges of the roofline. Just a few coats of epoxy, then paint? Or should I wrap with a light layer of glass too. It’s only 3/4”, so that’s a little tight for wrapping, but 4oz cloth should do it. Maybe I’m over-obsessing.

Opinions? Best practices?

Thanks.

Jbo

[Jaysen]

If you didn’t glass the top, it will be had to glass just the edge and make it smooth without a tone of fairing… which is my go to.

Run some cello/tape on the bottom to make a dam. Brush some neat epoxy, pack a pile of fairing compound on the edge. Pull the tape and sand.

It’s a bit of work but if you mix your own you can tweak the compound to be hard or soft depending on the edge. Also allows you to cover a lot of defects in the edge. Just be careful not to over sand and expose wood.

[jbo_c]

The top does have a layer of 4oz fabric(to, hopefully, prevent checking). My hesitation on wrapping with glass is just what you mention - I can see myself glassing only to then sand through while filling/fairing. So easy to do on a thin edge . . .

Jbo

[jbo_c]

Oh, and this is actual marine ply, meranti and okume if that matters.

Jbo

[Jaysen]

This won’t help you but…

I have discovered that it is easier to start with the edges. Put a good round over on the edge, strip of tape/fabric, then the flat. Lets you sand the edge tape but then cover the sanded section again. If you think about it, it makes sense.

Good luck.

[jbo_c]

Makes sense. I’ll do that on the other section of roof I have to do.

Thanks.

Jbo

[fallguy1000]

Exposed ply will fail. You are not overthinking it.

The reason this happens is there are varying rates of thermal expansion between the ply layers and glue layers and even sometimes a bit of water intrusion. Neat coating epoxy doesn’t stop it.

I recommend double wrapping that edge which means radiuses. In most glasswork 6 oz is enough, but you can end up with pinholes and track water easily. 4 oz can work. Wrap the uphill piece first. Grind/clean it up, then use thickened epoxy putty to fill any voids and same time wrap the final piece down.

It will fail and pop open if not done or done poorly. If any of the top is higher or flat, water will run to the wall; so you can also make a drip edge now, if needed. You basically add a piece of radiused ply, say 9-10mm with a radius on both sides to the bottom.

The front of my cabin gets a lot of water coming down it and I need to add ridges for gutters, and you could do that at this time as well, as or if needed.

Transoms are the biggest offender for this problem, but really, any exposed ply needs glassing to create a uniform surface for thermal expansion.

[fallguy1000]

Also, to prevent sandthru, be disciplined and only handform that edge. No sanders. You will see the glass and you stop then. Edges like that get filled with fairing compound after light sanding. You putty the flats first to fill the step you make with the edge glass. Use a white compound or colored compound, sand it flush to the top. Then use a radius tool to fill the edge and weave with compound. It can be done with neacoats, but takes too many if u ask me. If you were bright finishing, you paint the edge some accent color to deal with the fairing needed.

[fallguy1000]

Sometimes laminating the edges can be a pita if the glass doesn’t stay down. A couple ways to deal with it. You MUST always precoat exposed edges, so the radiuses you make get precoated and run the precoat up or on the down edges a way in and let them hot tack or gel. If you are careful, you can apply the glass to the sticky gel and wet it out stuck.

Or, you can use tape on plastic or pieces of peelply that are bigger than the wetted area and pull them tight.

For the top wrap, if you are close on space, you can also make some temp moulds or hold jigs with shipping tape on plywood bits and squeeze clamp any areas that won’t wrap.

I don’t like trying to wrap in a single go because glass hates making the full corner. But you can try it if you use peelply and tape and maybe some taped wood pieces and squeeze clamps. You may have a few spots fail and if so, make a putty with milled fibers or small cut fibers and fill as a repair, 2nd go.