Bubbles in Brushed-On Varnish

Brushes sometimes add air bubbles to varnish. Here are tips on preventing it. August 21, 2006

Can I put a layer of oil-based gloss polyurethane over gloss spar varnish (after light scuff sanding)? I have put 4 layers of varnish on a mahogany door, leaving more than 12 hours dry time between coats, working in a closed garage. Tiny bubbles appear as it dries. I know poly is a varnish and both are oil-based. I am hoping the poly will leave a smooth finish. Any suggestions?

Forum Responses
(Finishing Forum)
From contributor M:
Sometimes these bubbles appear when you brush too much. Try loading the bristle brush or poly brush, and then working with only one pass in each direction, and then use the other side of the brush in one pass in the opposite direction. Any time you use a reactive coating over another reactive coating, keep your fingers crossed.

From the original questioner:
I have been very careful about brushing slowly and as little as possible; loading the brush and gently pressing against the inside of the cup rather than wiping it over the edge to get rid of drips. Is there a non-reactive coating that would be better?

From contributor M:
No, non-reactive coatings contain strong solvents that may in some cases actually effect and react to the reactive coating.

From contributor C:
I bet that you are using a flagged bristle brush? These type brushes stir a lot of air into the varnishes. Instead, use a fine white bristled brush and tip off the varnish carefully with very light strokes. A little thinner may help you a lot, as it will let the varnish lay down better and allows the brush strokes to flow out better (gives you just a bit more time, too). I sometimes add just a tiny bit of refined linseed oil for better brushability and flow out... one tablespoon per quart is enough. Polyurethane varnishes are okay over standard varnishes. Though it is not an ideal finish design, it will work.