You will need to sand with fine sandpaper to dull the surface. Then wipe with paint thinner and follow with an ammonia and water wash to remove dirt and oils.

Then prime with an oil based primer such as Ben Moore Alkyd Enamel Underbody. Then finish with one or two coats of an acrylic semi-gloss such as Ben Moore Regal Acrylic Semi-Gloss.

Hope this helps out.

Yes

You have to sand the old finish then you can apply a coat on it.

Over a varnish i prefer to use NC clear coating or lacquer on it.

www.wisnofurniturefinishing.com

I would think that you would not want to apply a lacquer over a conventional polyurethane or varnish as the solvents will lift them.