You can paint over a surface with varnish on it. Sanding the wood with a random-orbital sander and 150-grit paper removes flaking-off varnish and creates a rougher surface on the rest of the varnish; this helps the primer bond to the surface better, explains The Family Handyman.
The next step after sanding is to brush away any dust. This provides a clean surface for the primer. The best color for primer is a light gray rather than white, so that the remaining varnish does not show through, notes The Family Handyman. After the first coat of primer, spackling compound keeps the wood grain from showing, so the second coat of primer leaves a clean look.