I have had similar experiences with different "urethanes". Varathane is a
Urethane modified alkyd. Some causes of "pinholing" are:
Poreous surface/substrate - material gets into the poreous substrate, the
surface of the applied paint "surface cures" (skins), then the trapped
solvents break through this "skin" after the material is past optimal flow
point. Adding more and more volatile solvents may compound the problem.
DuPont makes a product for the automotive finishing industry called Fish-Eye
Eliminator (flow modifier), It works quite well. Also most makers of
Urethane have "flow modifier" solvents.
Applying too thickly. Again, trapped solvents must breakthrough the surface
Excessive volume of hot or warm air will increase surface drying and
compound the problem.
Try thinning as per label, don't use heat or fans if possible or use them
indirectly (don't blow heated air across the surface). Also try putting on a
very thin coat (20% thinned) to fill surface porosity (?).
Also try foam brush or mohair applicating "pads". I have had good success
with them for small jobs.
Larger items are probably best sprayed.
FYI, I am a Journeyman Painter (28years), NACE Certified Coating Inspector.
I know of what I speak!
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