I am remaking a set of porch rail boxes for my 100 year old house. The boxes hold up the railings around a porch. The old ones rotted out from the bottom up by wicking up water (and just being really old). The boxes are of the post and panel construction type. I want to use regular 3/4 plywood as the panel. It would be more stable than solid wood as I could glue them without fear of expansion/contraction issues. I have lots of 3/4 interior plywood and don't really want to buy expensive exterior. The rail boxes would be painted. 40% of the plywood panel would be exponse. The rest is covered with trim. The rail boxes have a top and are open on the bottom. They are screwed to beams that protrude from the porch roof. The bottom edges of the wood have a vinyl edging (to prevent future wicking).
I would appreciate any thoughts with respect to the use of interior plywood in this application.