What Caused the Pits in a Varnished Tabletop?
I’ve been spraying McCloskey Heirloom Varnish for several years. From time to time, it develops small pits. It happened most recently on a mahogany desk. After sanding and applying wood filler and two coats of sealer, I built up coats of thinned varnish. The pits show across the entire top.
Richard Everett, Columbus, OH
I suspect that the wood filler is the cause of the problem. Most likely, it wasn’t completely dry before you moved on to varnishing.
The filler shrank as it dried, pulling the varnish down into the wood pores and creating those small pits.
Subsequent topcoats weren’t sanded flush, the filler continued to shrink, and each new coat of varnish magnified the problem. Your only recourse is to sand off the finish, let the filler dry completely, sand it, and start over.
Like you, I learned the hard way that oil-based fillers take a long time to dry. I give them four or five days to cure and shrink before I move on to other finishing steps.