Polyurethane varnish should be completely inert once cured. Prep the wood well then paint and varnish and everything should be fine. Although prolonged direct contact with the water should be avoided the odd drip from condensation will be ok.
Previously I have coated the inside of a DIY wood with tin foil wallpapered on with PVA glue. This held up well and acts as a reflective surface for lighting.
Thanks for the info! after posting this i had just been told about the polyurethane varnish, and i had some offcuts from the build left outside that i had put a few coats of spray paint on yesterday to see how it looked. i just put a coat of varnish on it as a test.
just want to clear a few things up as im afraid know almost nothing about working with wood.
1. you said prep the wood before painting, i assume this means sanding it down to prime it for the paint to stick on?
2. the other thing was you said it will be inert once 'cured' am i right in thinking this just means allowing it to dry?
1. Yes, sanded, cleaned, primed just the usual really. Seems obvious but certain people I know have just opened the tin and splashed paint on greasy wooden kitchen doors and then been surprised as it flakes off all over the place for the following months!
2. Cured is fully dry yes. Paint is often touch dry in a few minutes or hours but that's just the surface. Enough to put a second coat on perhaps but not dry right through so it can be easily damaged and is still changing chemically. Curing takes from 24 hours upwards depending on the paint type and atmospheric conditions. Personally I'd leave it about a week.