Forgot to mention ........
For ultra-quality varnish and other coating work and before the actual
coating I sometimes take ultrafine pumice and rub it dry on the surface
of the wood **** to fill the pores**** of the wood. The result is a
smoother top surface of the wood and the pores fill up with the pumice
fragments .... which results in less 'out-gassing' bubbles during the
first coats. You do have to be careful if the particular piece of wood
has many large pores as the accumulated pumice will 'show' through
under the varnish. It works for tight grained wood surfaces where
there arent many surface 'tear-outs' and 'pores' that need filling.
Works better than an acrylic surface sealer, which sometimes yield a
'milky' hue to the wood surface. Surface filling is a trade-off as
you get a flatter surface, less out-gassing from the 'pores', etc.;but,
lose a bit of the adhesion of the coating to the wood. If your 'prep'
is just flat sanding and you dont fill the pores by either using a
sealer such as acrylic or pushing in pumice, etc. , you can always get
'bubbles' in the first few coats.
If youre in a hurry to varnish, employ the 'six foot rule'.... if you
see a defect in the finish develop, step back 6 feets from the surface
and if the defect disappears due to the distance just continue on. In
reality noone looks closer than 6 foot to a super finish.

Hope this helps.




In article , Rich Hampel
wrote:

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.