I have had similar experiences with different "urethanes". Varathane is a
Urethane modified alkyd. Some causes of "pinholing" a

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
skin.

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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"Ken Moiarty" wrote in message
news:tcwif.632086$oW2.346600@pd7tw1no...
I've been trying to do some sprucing up some assorted fixtures,
moldings, and other miscellaneous household surfaces by brushing on white
Varathane 'Colours In Plastic' (oil based polyurethane) paint. However,
I'm finally throwing in the towel as far as using a brush with this stuff
is concerned: Can't seem to avoid the creation of little pin-head sized
bubbles everywhere. Most of these bubbles eventually pop on their own,
but always too late in the setting process not to leave tiny pimple-like
craters scattered everywhere, simply making for an unacceptable finish.
I'm going to have to switch to applying Varathane paint by spray from here
on. (I've discovered there's no problem with bubbles when applying the
Varathane by spray.)
But spray painting means having to move or remove whatever I want to
paint with this stuff into an area suitable for spray painting and this
just isn't practical for every surface I would like to touch up.
Therefore, I'm looking for suggestions as to other products and/or other
types of paint I might turn to that are like Varathane insofar as they
produce a smooth, hard and durable enamel-like coating, but without the
problem of countless micro-bubbles forming when being applied by brush.

TIA,

Ken