Luan is usually a cheap ply, there are indoor and outdoor grades.

"You can get exterior luan (meranti) with phenolic glue. It will have the letter p before the grade (usually bbcc). You can also get pressure treated marine ply."

Here is a link that might be useful: Difference between varnish and shellac

I just painted the ramp with spar urathane it just dried and seems to be working it just started to rain i i put a tarp over it to ensure it gets the recomended drying time

how much is exterior luan (meranti)the stuff i bought was $13 at home depo a sheet