balsa wood part

I'd paint it first with several coats of gloss paint.

Also depends on the part shape, no negatives would be helpful

You would probably have way better luck with basswood,and as gmorrison suggested ,heavily sealed. I would probably seal it first with some type of sanding sealer then  a coat of  gloss varnish

thanks to all will give it a try

I always put a glossy coat on the wood part before trying to make an rtv mold of it. If there is a lot of detail, especially indentations and narrow grooves, I may put a mold release on it.  Talcum powder makes an okay mold release, but a mixture of vasoline and paint thinner is even better.  For the latter, however, make sure paint is thoroughly dry on pattern.

I would seal it, I never have issues with damage to the master unless its super fragile.you could make a multiple piece mold, by using vasaline in between the two halves. I make alot of small very detailed parts for 1/25 scale trucks.