Edges Cracking On Outdoor Plywood

I live in central Michigan and am looking for some help. I build a lot of outdoor wood decorations and this past year I started using 3/4 ply instead of solid pine. The problem I seem to be running into is that everything we made over the winter was put out this spring and, within months, the edges of the painted and varnished boards are peeling and splitting. To give you a background, it’s 3/4 pine plywood; all have been shellacked, painted with latex craft paint, and three coats of G.F exterior 450 and allowed to dry for weeks. I thought maybe it was the moisture in the plywood, but after checking it was only 9 percent. Please help! – Brock

Chris Marshall: Brock, despite your considerable effort to seal out water, seems like the effects of sunlight (UV rays, heat) are still causing the exterior varnish to crack and water to penetrate through it. Once water gets in, it’s soaking through the paint and shellac and into the end grain of the plys. The plywood is getting damp and staying that way, and that’s what’s causing the finish to peel and the plywood edges to split. The solution? Make sure you’re using an exterior-grade plywood (B-C grade should work, or marine-grade) to start with, which will be made with water-resistant glue. That should help minimize plywood deterioration. Or, if solid pine worked before, switch back to it again. I’d probably use an exterior latex primer and paint on top of the plywood or solid pine and just leave it at that — no varnish. Without the varnish layer, the plywood might also dry more quickly and thoroughly after it gets wet.

Posted in: