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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. |
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