How do I waterproof plywood?

[Fat Charlie]

I have trouble using brand new paint successfully.

[samwichse]

You would probably save yourself a lot of grief just getting the marine-grade stuff.

[Xist]

This shows over $200 for a 4x8 sheet of plywood, mostly in increments of 1/4". Our trim is about 3/4" thick, but a 1" board is only about half an inch thick. http://www.boulterplywood.com/MarinePlywood_4.htm

I called a local lumbermill and they said they would come out and measure, then custom-cut boards for me. Would that cost more than two hundred dollars?

[RustyLugNut]

and prime/soak exterior grade ply with Thompsons or an equivalent and then use an oil based Kilz to prime after a thorough few days to flash off. This will allow you to paint with an oil based paint for maximum protection over decades in the Arizona weather.

Cut your ply to widths you will use so you can treat your edges as well as your faces.

[roosterk0031]

1x4 board should be 3/4" thick, for example a 1x4 is 3/4"x3 1/2". 1x6 is 3/4 X 5 1/2, but a 1x8 is 3/4x7 1/4.

Dimension under 2" are a 1/4" undersized, 6" down to 2 are 1/2" undersized, over 6 are 3/4" undersized.