Try as I may, I have had trouble with tiny bubbles in the application of my water-based polyurethane varnish. I carefully prepared the surface with 220 grit paper ans used a tack cloth to remove any residue. I bought a new can of MINWAX varnish, a new brush (specifically for varnish); did not shake the varnish in any, but stirred it carefully; applied gently and in one direction. But these tiny bubbles occur and dry in the varnish. Any suggestions?