which is the best way to seal the wood surface before making the mould ?

Not sure what the 'best' way is but i can tell you how I've done it.

1 way is to paint it with car paint... not the rattle-can stuff but proper 2-pack, the filler primer type is the one to go for... this builds a thick coat coat that's pretty easy to sand and polish. you just stick you release agent onto that and you good to go.

the other way I have used that worked pretty well was to use shellac sanding sealer, I think it's normally used for furtinture and stuff, you can get it from B&Q, a few coats of that and a sand and buff and again you can go straight on with release agent.

basically I think anything that seals the surface and allows for a bit of a polish will work as long as it doesn't react with the resin, always try a test peice beofre you go and get your pattern stuck in the mould!