Any ideas what is the best thing to use to paint the underside (roadside) of a wooden floor based kit? It`s probably marine ply.

I would consider sealing it with a sealant used for concrete ponds (available from most aquatic stores) and then a couple of coats of a decent polyester top coat

Or just use an exterior grade paint from DIY store if you are feeling locost

[Edited on 28/4/13 by tegwin]

Surely Yacht varnish??

I want to paint it black to match under the chassis. One suggestion is a coat of resin to seal it first and then paint on top?

Resin is very brittle so not a good idea!

Poly top coat is slightly more flexible...

I'd seal it first, then apply a coat of car body under seal. The black stuff that stays soft. That would help protect it from stone damage as well.

Underseal dries out and cracks if your going to use anything like that try some stonechip which dries out but stays flexible