I'm not quite sure what you are trying to do. If you are trying to create a distressed shabby chic look then you would be better off using a chalk paint rather than gloss. If you want to paint gloss paint onto sanded varnish then gloss will normally go straight on but If the furniture has been waxed then there may be some residue which would resist paint. If you want to be on the safe side you could prime it with a coat of Zinsser BIN which is a special type of primer sealer which will stick to almost anything.

I hope that helps