While not very easy to apply by spray application, oil based varnish is one of the easiest finishes to apply by brush. Because varnish sets-up slowly it gives the user plenty of time to brush and spread it out evenly on to the surface. It's hard to spray because it has a tendency to run if applied too heavy. I firmly believe that any film finish can be sprayed successfully if thinned out enough, but varnish is one of the last finishes I would want to spray. Over many years of testing, I have come to realize that brushing is the best way to apply oil based varnish.
Before applying varnish by brush, you should know a little more about how long it takes for each coat to set-up and how long before you can apply the next coat along with how it reacts to temperature and humidity and some other facts. Oil based varnish is much higher in solids than some other film finishes like lacquer. Therefore, it should only take a few coats of varnish to build a film significant enough to protect the surface of what you are finishing. After the surface has been sealed, it usually only takes about three coats to give you enough protection.
One very important factor when applying varnish is how the temperature effects the speed at which it cures. You should not apply varnish in temperatures lower than 65 degrees. If you apply varnish in lower temperatures it may take several days, even weeks for it to cure. Room Temp. (approx. 70 to 75 degrees) is good for applying varnish. Hotter temps. will make the varnish cure quicker, but the solvent in the varnish will evaporate quicker, making the varnish set-up quickly and you may have a problem getting the varnish to flow out properly. This could result in brush marks, bubbles and an uneven film. When working in temperatures higher than 75 degrees, try not to work on large surfaces.
Try to set aside a room or part of your work shop to apply your varnish. This room should be as dust free as possible. Do not do any other woodworking, (especially sanding) in this area. If you are going to set aside an area of your shop instead of using a different room, it would be a good idea to also surround this area with heavy plastic sheeting. Before applying the varnish, wet mop the floor, this prevents you from kicking up any dust when you walk around. I always place clean craft (brown) paper under the piece I will be varnishing. Once the surface has been prepared properly you are ready to brush on your varnish.
There are a number of high quality brushes that can be used for brushing on clear topcoats. The best for shellac and lacquers are natural hair (like badger) or china bristle brushes. While any of these brushes will do a great job when applying varnish, there is a much less expensive alternative. A polyfoam brush. That's right, the disposable type. Oil based varnish is classified as a cold finish. This means the solvent use is not as strong as evaporative finishes like shellac and lacquer. Alcohol and lacquer thinner will melt a foam brush but the mineral spirits, solvents or turpentine used in most oil based varnishes will not harm a foam brush. Foam brushes are especially useful for novices who have a hard time getting brush marks out when applying a finish. If used properly, you can get excellent results. I always have a good supply of 1",2" and 3" foam brushes in my shop. They are very inexpensive, so I use one for each coat and then throw it away.
You don't need a special sealer to seal the wood. Special sealers like sanding sealers will not do any better of a job of sealing the wood than the finish itself. Sealers only make the first coat easier to sand, thus speeding up production time. Also, if you use the wrong type of sealer, you may have adhesion problems. The best sealer for your first few coats should be the varnish itself. Take some of the same varnish you are planning to use as your finish and thin it down 50 percent ( this is a 1 to 1 ratio) with mineral spirits or gum turpentine. This will be your sealer. It will do a good job of sealing the wood and you won't have to worry about contamination problems. Pour some varnish through a paper paint strainer or stocking into another can or jar, then add the same amount of mineral spirits into the varnish. Stir well and strain a second time into a deep dish or bowl. It's best to work out of an open bowl or dish so you can easily dip your brush into it. Now, dip the foam brush into the mixture until the brush has been loaded slightly past the bevel on the foam brush. Lift the brush up and let the excess drip back into the dish. Next, brush on the first coat with the grain making sure not to leave any puddles or drips. Allow the sealer coat to dry overnight and then sand with 320 grit paper. Remove the dust with a vacuum, or tack cloth. If you are working on very porous woods, apply a second sealer coat following the previous steps.
It's a good idea to also thin out your coats of varnish a little. You can reduce your varnish 20 to 25 percent 4 parts varnish to 1 part mineral spirits or gum turpentine or 3 parts varnish to 1 mineral spirits or gum turpentine. This will not effect the strength of the varnish, it will only make it flow better and allow time for air bubbles that form when brushing to pop. The only drawback is that you will have to add a few more coats because less will remain on the surface once the varnish has dried. Prepare the varnish by mixing and straining in the same way you prepared the sealer. Use a foam brush and load it in the same manner as the sealer. Apply the varnish to the surface by brushing either with or against the grain initially. The main idea is to get it on the surface doing as little brushing as possible. Once on the surface take one light pass with the tip of the brush moving with the grain. Overlap each pass slightly, then leave the varnish alone, do not do a lot of brushing, this will make the solvent evaporate quicker and the varnish will set up too quickly and not have enough time to flow out. Let the varnish dry overnight, and then sand with 320 grit sandpaper and remove dust using vacuum or tack cloth. When sanding, if the varnish starts to clog the paper, it has not dried enough. If the varnish turns to powder, it is dry enough to sand and apply the next coat. Continue to apply 2 to 3 more coats of varnish using the same process. If you are going to rub out the finish (by wet sanding) after it has cured, you may want to apply at least a total of 6 coats ( not including sealer coats). This is because if there is not enough varnish left on the surface, you may cut through the finish into the raw wood in some spots.
Once you have applied the last coat, let the finish cure for several weeks before you are ready to use it or rub it out. Varnish does not need much maintenance. If you wish, you may apply a coat of paste wax or liquid polish from time to time.
If you want to rub out the varnish finish and are not familiar with the process, I am planning to put this info up on my homepage soon. However, if you need this information sooner, or have any other questions, please contact me by e-mail.
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