Can I apply polyurethane to a new table?
(See my above comment) Unfinished or stained wood is best for a poly finish. Have you considered getting a clear glass top protector for your tables? 1/4" glass is pretty cheap and you can get it cut to size with a beveled edge. Affix it to table with clear adhesive pads--very cheap and easy to use. Going with glass will be MUCH LESS work and will last forever (pretty much).