I have a exterior hose reel that attaches to a wooden mounting plate, the plate is attached to the side of the house with standoffs and the holes the standoffs fit though in the vinyl siding are sealed with foam. The problem has been the wooden mounting plate -- I've made three over the last 10 years and Mother Nature/water ALWAYS gets the best of me. It doesn't matter what material, design, coatings I use; eventually my plate fails through joint failure (whether the joints are in between two boards, or I've used plywood with hardwood edge treatments attached with biscuits and Gorilla glue; that also has failed after several years. So this time around, I'm going to use PVC.
I bought a PVC trim board at the home center. In order to make a plate sturdy enough, I'll edge join two pieces about 12" long and attach another pair of edge joined pieces rotated 90 degrees. For appearance only, I'll probably put a small radius on each corner and round over the edges. For paint, I'll use an oil based primer and two or three coats of exterior latex.
But what is the best material to join the material both on edge and when lapping the 2nd piece? With regard to edge joining, should I also secure with pocket screws? What material should I use to fill any gaps?