Paint is easy because it doesn’t have to go on perfectly; brush marks and other minor flaws are expected and accepted. Varnishes are easy because they dry slowly, so there’s plenty of time to spread them out evenly and get the brush strokes lined up with the grain.
Polyurethane varnish is the finish most widely brushed by amateur woodworkers, and it is the finish that is almost always used in instructions on how to brush. (See “The 7 Myths of Polyurethane” in the finishing section at popularwoodworking.com.) There are countless articles, several videos and even segments of television ads that describe or demonstrate how to brush varnish.
For a reason I don’t understand, almost all of these instructions suggest or show brushing slowly, several at the incredibly slow speed of eight seconds per foot. (Try it; I’ll bet you can’t brush this slow the first time).
There was even a recent TV ad from a major finish supplier that showed brushing the entire width of a tabletop only 1′ in from the end of the boards (to be followed presumably by the next foot, which would leave a distinct overlap) instead of using long strokes running the length of the top.
Though this method is counterintuitive, and most people will figure out very quickly that it produces unacceptable results, the authority presented by TV and a brand-name company can’t help but lead some people astray. (For example, a senior salesman from a major paintbrush manufacturer came to my shop once and used this method while we were trying out different brushes.)
Though it’s also somewhat counterintuitive, many instructions suggest brushing across the grain, or diagonal to it, before lining up the brush strokes with the grain. Other instructions caution against shaking the finish, or even stirring it, because this will introduce air bubbles.
Brushing is very simple, actually intuitively simple. It shouldn’t require a magazine article like this one to put common sense back into the process.
The Basic Rule
The most critical rule for achieving good results, and the rule you almost never see, is to watch what you’re doing in a reflected light. This rule holds for spraying just as it does for brushing.
You can see what’s happening in a reflection, and you can’t see if there isn’t one. As long as you see a problem as it occurs (usually a run, sag, drip or bubbles), it is usually easy to fix simply by brushing back over the surface.
Bristles or other trash that may have fallen into the finish are also easy to spot in a reflected light, and they’re easy to remove with the tip of the brush or a small tool such as a toothpick. The damage done to the finish can then be repaired by brushing back over.
No matter which finish you’re using, the procedure for brushing is the same. The only difference is you have less time with fasterdrying finishes, such as shellac and waterbased finish.
On any given object, begin by brushing the least important parts first. This way, any overlapping will occur on less-seen parts. Tabletops, chair seats and backs, and cabinet doors should be brushed last.
Be especially careful of runs and sags on vertical surfaces. Watch the surface in a reflected light as you brush, and brush the finish back out flat if it begins to sag. If you’ve applied too much finish to get it to hold to the surface, use your brush to transfer some of the finish to another part or remove the excess finish by dragging the brush bristles over the lip of a can or jar.
It doesn’t make any difference in which direction you brush – with the grain or across it. Brushing across the grain doesn’t help the finish get into the pores; it soaks in quite well by capillary action.
Whichever direction you brush to begin with, however, be sure to brush back over and line up the brush strokes with the grain if possible. It will help disguise the brush marks and knock off most of the bubbles, if there are any.