Howard Acheson
>>>> Baltic Birch ply is not made with waterproof glue.

Actually "Baltic Birch" manufactured in Finland is made with exterior rated adhesive. It's called Finnish Birch. Russian make Baltic Birch is made with interior rated adhesive.

>>>> then get plywood designed for outdoor use, or the special MDF used for signs.

I think you meant "MDO" Medium Density Overlay. That's the material used for exterior highway signs. MDF will not stand up to experior conditions.

>>>> The paint should protect from normal UV/rain. I do not think the Varithane on top of an exterior paint would help, and may not adhere depending on the type of paint. I like Varithane, but it will not stand up long for outdoor use due to UV.

Correct. UV will rapidly attack urethane resin finishes causing it to get cloudy then begin to crack and peel. I good exterior rated paint is more long lasting.