In order to put a dog door into the back yard, we had to "extend" the back porch a couple of feet.  Originally, we used some primed & painted OSB, but its starting to delaminate (about seven years).  I'm looking to replace it with some 1/2" marine plywood.  I'm going to prime it well (especially the edges), and paint it with some outdoor porch & step type paint.  My question is what to do with the edges.

One thing I've thought about is putting come aluminum c-channel to wrap the edges to help keep the moisture out.  In order to provide a little extra protection, I've also thought about putting some silicone caulk into the c-channel before attaching it to the plywood to help with waterproofing it.

Any thoughts?

You could try putting a skim coat of silicone or urethane sealant on the edges.
Just enough to seal the pores should work.

Why not use something like a hardi panel?  Cement based panel made for weather exposure.


True marine plywood is probably overkill.  Its different in that there are no voids in the interior laminations.  If you end up using plywood, use a good exterior grade plywood and seal the edges.   A cap over the edges seems reasonable.

This is what I do, works fine.  Make sure you get a paintable caulk, though.  100% silicone looks like hell if you try to paint it.
Really no need for marine plywood; pretty much anything you find at the local lumberyard/big box store will work fine if finished properly.

That is the best option.  IMHO

Marine and exterior plywood share very similar construction. Marine plywood is guaranteed to have no internal voids.

You could treat the plywood like they do in the marine industry. They use epoxy and paint both side and all 4 edges with it before installation. Then they glue it down when possible, but if they have to use screws they pre-drill, add silicone sealant, and then drive the screw home. This makes a piece of plywood almost impervious to water.

We used fiberglass resin to seal MG plywood at the marina I used to work at. Once resin has set up (over night) scuff it up with some sandpaper before painting with porch paint.

^^^ What I came to suggest.