You need outdoor (exterior) plywood. Ask your lumber yard people.

My guild is having someone come in to do a workshop on this in October! We will be painting a 24 inch square block. Sorry, but I have no idea what type wood we will be using, the instructor will bring everything we need with her on the day of the workshop.

I use 1/2 exterior plywood & prime it on all sides... I transfer my design on with pencil lines & sometimes I mask with tape & spray paint & on others I paint with brushes up to my lines....here are a few of mine

Oh....forgot to mention size....mine are 24x24 & 36x36 mostly....I have done small ones too 12x12 (birdhouse attached)

My son can get me as much Russian Plywood as I need.

I'm from Wisconsin, and our winters can be brutal. I would suggest using a Marine Varnish. It stands up to all kinds of water
Post a pic when done....we'd love to see!

I have been thinking about doing a barn quilt with a cloth quilt block glued onto exterior plywood then
coated heavily and completely with polyurethane sealer. What do you think?