Glen probably has the answer to your problem.

Porosity in the wood, even with several prior coats of varnish, will
sometimes make bubbles as the temperature changes. The use of a 'spit
coat' as the *first coat* usually solves the problem. A spit coat is
usually mixed as 40%-50% varnish and 60%-50% thinner appied heavy and
'worked-into the grain' with a brush. The extra solvent will greatly
'thin' the mix so that it will penetrate into the intersticies of the
wood cells. An alternative is to use a 'sealer' - usually a clear
penetrating acrylic that you apply, let cure, then lightly flat sand.
Sealers are not really a good idea as they are not UV stable and
enhance 'lifting' when the sealer eventually degrades in UV ---- better
to use a very thinned-out 'spit-coat(s)'. When I was a
'varnish-addict' I'd wait for the hottest time of the day to apply the
spit-coat : let wood get warm, then put up a shade so that the wood
begins to cool and apply the spit-coat on the 'cooling' wood. If the
spit-coat is warmed a bit, the better the 'penetration'. Also, watch
the barometer as a falling barometer will aid in 'off-gassing' during
varnish application.

I used to prefer(ed) to lay down the first few (4-5) coats with tung
oil based varnish for 'penetration' , let fully cure; then a few coats
of urethane based varnish for UV protection, let cure a month or two
then apply a 2-part clear (Interthane, etc) on top of that for
durability ..... and 3-4 years later remove it all when the first coat
lost 'adhesion'.

I gave up using brushes and found that using a small airbrush is vastly
superior ..... and then I discovered the modern 2-part acrylic/urethane
co-polymers with heavy UV protection that last 5-6 years; and, havent
opend a can of 'exterior' varnish since.