Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
If you use a deck sealer most suggest reapply every few years, or even annually. Most of their effectiveness is based on 2 ingredients: Oil to repel water (Linseed was the old one, maybe now they are using something else) and a UV protecting material. Neither of these are something that is useful in this situation.
If you want this effect, look into hardwood floor finishing products. I have made several stands and finished some with stains, and some with clear coating only. I did some plywood stands, but most are lumber.
For plywood I would use a high quality exterior paint with the longest warranty I could find. Does not need to be oil based, but it could be, if you can do the work in the garage or outdoors. Acrylic paint is not fun indoors, but is do-able.
Plywood is not usually so interesting that I would want to use a clear sealer sort of material.
Of course caulk, fill holes as needed before painting. Use all outdoor rated products.