There is foam made just for marine upholstery applications. Your best bet is
to Google 'marine upholstery supplies'. There's lots of mail order places
but with luck you just might find a store near you. You need a foam that
will drain or not hold water and is mildew resistant. I'd also suggest
drilling a few holes in the plywood so you can have a little better
circulation.

a for instance: http://www.garysupholstery.com/uphol...m.html#outdoor

Val

"SteveB" wrote in message
...
I want to make a bench seat for a boat. I want to cover it with Sunbrella.
Just put piping around the edge, and wrap the canvas around the foam and
staple it under the plywood base. What type of foam would be best to use
for outdoor application. No direct sunshine, but lots of heat, and getting
wet and drying out. Something that won't turn to dust.

Steve