Many people don't have spray painting equipment, so I am posting a "how-to" based on my own experience:
You can get a seriously good smooth finish with polyurethane paint using a brush if you do the right paint prep, and have a bit of patience to get the technique right. Interestingly, most people have absolutely no idea how to use a brush correctly.
Prep work (Needs to be done at least 6 hours before you want to start painting)
1. Sand the substrate to a fine finish
2. Clean well
3. Mix up the polyurethane in a ratio of 1:1 with turpentine (it cures slower, and allows for a nice thin coat when cured). Your paint must have the consistency of water. This goes against most of the things you will read on the tin, but trust me on this: thick paint does not work well at all.
4. After mixing the turpentine into your paint, close it up and leave it to stand for at least 6 hours - this ensures that all the air trapped in the paint mixture surfaces and is released.
5. Clean your brush well. Ensure that no bristles will come off when you are painting. Use a steel wire brush and run it through your paint brush a couple of times to get rid of loose bristles.
Painting
1. Dip your brush in turpentine so that the bristles are covered in it entirely.
2. Open your paint mixture (do not shake it at all. If you do, you will need to leave it for another 6 hours to degass) and very gently stir it with a flat paddle so as to not get any excess air in there.
3. Shake the excess turpentine from your brush
4. Dip the brush into the paint so that about 60% of the brisles are in the paint.
5. Gently depress the brush on the side of the paint container to get excess paint off. Again, the idea is to get as little air into the paint as possible.
6. Holding the brush at a 45 degree angle, and apply very slight pressure to bend the bristles slightly as you apply the paint to the substrate. You need to get a good, wet coat with one single stroke of the brush.
After each coat has cured, sand it smooth with 400 to 600 grit sandpaper, then clean, and paint again until you have the finish and depth you require.
What to do and not to do
* If you need to apply any more pressure to the brush in order to get the coat on, it is time to refill the brush. Repeat from step 4.
* Do not go over any area where you have applied paint. Doing so will introduce bubbles in your finish.
* If something happens to the finish while wet, like a bug or dust getting stuck, or a bristle falling out, then leave it there. Do not try to remove it as you will spoil the finish. Rather sand it out when cured as it is much less invasive.
* Never, NEVER go back and forth with the brush. It will introduce bubbles and spoil your finish.
* If, after painting the entire surface you find some small bubbles in your finish you can try to gently blow on it using a straw. This will generally break the surface tension and allow the gas to escape, and the finish will flow over the area to cover it. You need to do this within the first 10 minutes, or otherwise leave it to cure and then sand it out.
* If you have a paint run, do not try to fix it. Leave it to cure, and sand out after.
* If you are painting wood, you will find that the first coat will be very rough after it has cured. This is because the paint hardens the grain of the wood and makes them stick up. After applying the first coat and leaving it to cure, sand it down using 400 grit sandpaper until it is perfectly smooth. Your second coat should be much better in terms of the texture, and your third coat should be perfectly smooth.
* If you are painting on wood which shows the grain (ie, you are applying a clear finish on wood), then always sand with the grain - never across it. By doing this the grain "hides" the sanding marks. If you sand across the grain the sanding marks show up like a sore thumb, even if you use high grit sandpaper.
General overview
1. Air in the finish is not your friend. Take all measures possible to minimize any air getting into your paint.
2. Paint using a single stroke only
3. Do not press down on the brush
4. Do not try to fix anything while the finish is wet. Do it after the finish has cured.
I have done this on numerous occasions, the most famous being the maple speakers (with black epoxy lines) using the Seas Excel drivers.
It takes some patience and a little practice, but the results are really good. If you are careful you will get as good as a spray finish using this method.
Good luck and most importantly, have fun. It shouldn't be a chore, but an effort of love. The more you care about the finish, the better your results will be.
Regards,
Ian.
