Why Do I Get Bubbles and Brush Marks In My Varnish and Polyurethane?
By Sal Marino
Every day I receive a great amount of e-mail. By far, one of the most frequent problems readers e-mail me about is how to eliminate the bubbles in a finish that they have brushed on to their project. I do address this problem in the article I have on my page titled "Brushing Varnish". However, I do realize that some readers do not read every article on the page, therefore, I have decided to put up this small, separate article in the attempt to both help the readers who are having problems with this and reduce the amount of e-mail I receive on this subject.
What Causes the Problems
It is very frustrating when one spends hours and hours building a project, making sure that all the joints fit properly, the wood grain matches from board to board, removing any glue squeeze out, etc. only to wind up with brush marks and or bubbles in the finish they have brushed on. Most people immediately blame the brand of finish they have used and swear never to use that inferior product again. In truth, the problems are not with the brand of finish. Most popular brands of varnishes and polyurethanes will give you excellent results if you know how to properly brush on that finish and a little about how finishes flow out and their set up time.
There are two main factors that cause a finish to bubble and show brush marks after it has dried. The first has to do directly with the way you apply the finish, (your brushing method). Many people assume that if they brush out the finish well, it will level and stay smooth. This is absolutely incorrect. When you apply a finish by brush, you are basically agitating (moving the finish around) Brushing introduces air into the finish, thus creating air bubbles. Even if one is careful, one cannot completely avoid the development of some bubbles, but if you brush too much, you will wind up with a whole surface full. Secondly, the more you brush out a finish, the quicker the solvent in that finish will evaporate, thus the quicker it will set up. If the finish sets up too quickly, it will not have enough time to level itself. The bubbles will not have enough time to pop and any brush marks will not have enough time to flow out.
How To Prevent the Problems
Thinning the finish will make it flow out better so brush marks will level and air bubbles will have enough time to burst before the finish sets up. First, make sure the finish has enough solvent in it. Most oil based varnishes can be thinned with pure gum turpentine or a good quality mineral spirits, but it is always a good idea to check the label on the can to make sure. I usually like to thin out the finish about 20 to 25 percent with solvent. This breaks down to 1 part solvent to 4 parts finish or 1 part solvent to 3 parts finish. Thinning this much may not be necessary when the can is brand new and it's the first time you have opened it, but each time you use the finish, it will get thicker because as the amount of finish gets lower in the can, the more air stays in the can and this will make the solvent evaporate much quicker. If you are not in the business, I do not recommend buying these finishes in large cans like gallon sizes. Purchase in smaller sizes or when first purchased, transfer the finish from the larger can to smaller ones and make sure the lid is placed on tightly. This will keep the finish from thickening too quickly.
Reducing the finish with solvent will not make it weaker or effect the way the finish performs. The only drawback is that when the finish dries and all the solvent has evaporated, less of the resin (finish itself) will be left on the surface. Therefore, you will have to apply a few more coats than you would have if you used the finish without thinning it. I believe this is well worth the extra time, considering the results you will obtain.
Proper brushing technique will reduce the amount of air bubbles that develop in the finish. When brushing on the finish, make sure to load the brush enough so you can apply a liberal amount of finish without having to press down on the brush to get more out. Lightly touch the surface with the loaded brush and then start to move the brush across the surface. As soon as the brush stops applying a continuos flow of finish, re-load the brush, in this way you will prevent over-brushing. Try not to overload the brush, because if you do, you will wind up with drips or too much material on the surface. Some practice is required here. Run tests on scrap panels until you get the feel. Once the whole surface has been covered with finish, then you must perform a method called tipping off the finish. Do not re-load the brush. Using the very tips of the brush, take light passes just slightly overlapping each brush stroke. This should burst the majority of bubbles. This will not burst all the bubbles, but don't worry, the remaining bubbles will burst and level because of the extra solvent you added to the finish. Remember, sand lightly between coats with 320 grit paper.