Brushing Polyurethane

Bruce Hamilton replies: The source of bubbles that form in your varnish film as you apply it may be caused by a number of things, however it has been my experience that the primary cause is due to an incorrect application technique.

Varnish needs to be flowed onto the surface. Dip the brush into the varnish for about half the length of its bristles. I first touch the surface in the middle of the board and flow to the right with the grain. Then before the load of varnish on the brush is spent, I return toward the middle and flow past it to the left.
Depending on the rate at which the varnish is beginning to dry or stiffen, I may apply two, three or four more parallel lines of varnish before returning to the container I’m varnishing from to remove any excess material from the brush.

It is best to varnish from a container other than the one the varnish came in. I like to put a piece of wire across the opening of my application can to rub the excess varnish from my brush for the next step. This is traditionally called “tipping off”. To tip off the varnish I’ve already applied, I hold the brush lightly on the surface, slightly off the vertical and drag the brush through the varnish. Excess varnish will be drawn up into the dried brush which can then be removed on the wire in your varnish can.

Repeat this step once or twice before proceeding to the next area to apply more varnish. This tipping off step evens out the varnish coat, and more importantly, disturbs the surface just enough to let any trapped solvent or air to be released before the varnish begins to stiffen and skim over.

One of the problems with the newer varnishes is they dry too fast and skim over before you can tip off and release trapped solvents and air. To solve this problem, some application techniques suggest chilling the varnish before you apply it. That, however, is a whole other discussion with its own pro’s and con’s. One final note, don’t hesitate to call the manufacturer if you are having problems with a product.