Water Stained Table Won't Take New Varnish

Were you trying to use an acrylic varnish or other water-soluble varnish? If so, the oil in mayo would repel the varnish. Try using a solvent-based varnish (the type where you need to wash your brush with turps). This should work.
Brenda in Oz.

The table was light oak not painted black. The spot was black. Thanks