Furniture, Wood and Cabinetry Finishing - waterproofing antique sideboard for bathroom

I purchased an old sideboard to use in my bathroom. I have installed sinks in antique pieces twice before, but each time I stripped the piece and used polyurathane. I would estimate this piece is about 50-70 years old. I am afraid it is varnished and I don't really want to totally strip it because it is in good shape. Can I sand it and spray poly urathane on top of the varnish? Do you have any other suggestions on water proofing it?

Bibi:

My primary concern would be moisture absorption in the normally unprotected areas of a wood piece going into a high humidity environment - the inside.

But to address your question, unless the piece has been finished after it left the factory, the finish is probably lacquer. You'll have little luck applying a polyurethane finish over that. Check the poly you intend to use. It may allow shellac as an undercoat. If that's the case, you can apply a coat of shellac to the entire piece, then polyurethane over that. Shellac is compatible with lacquer AND varnish, and will serve as a barrier between the old finish and new - sort of like a paint primer.

Whatever you finish with, go over the entire interior of the piece as well. This will prevent, or at least slow down moisture absorption within the piece and keep it from swelling where you don't want (or need) it.

Thank you so much. I will find a poly and shellac that will work the way you describe. Would you suggest I use a spray or use a brush? Spraying is easier but I am not sure it covers as well. Recently I sprayed some chair legs and they ended up with whiteish areas on them. I have been refinising furniture for years and that has never happened to me before. Thank you again for your help.

Bibi:

Brush on finishes always go on heavier than spray becasue of the nature of the application.

Those 'whitish' areas were cause by moisture trapped in the finish before it could evaporate. Excessive humidity can cause this quite easily, as can working in normal humidty and lower than suggested temperatures.

You can always do what I do - cheat. Use a brush on product to build up the finish, sand it smooth and use an aerosol to complete the job. No brush marks to worry about, and you know the finish is heavy enough to provide adequate protection.

You are the greatest. I will let you know how it comes out!