Get a Perfect Top Coat of Polyurethane on Your Wood
After you have finished staining or painting a wood project you will usually apply a top coat of polyurethane to protect it from simple nicks, dings and scratches to harsh weather conditions. Applying polyurethane has its own issues that can cause your finished job to look bad. It can drip; streak and pool in areas or you can miss spots with it and not really know it. All of these things can be avoided when using polyurethane and following these tips and tricks.
Start with a Clean Wood Surface
Many initial problems with applying polyurethane to wood is the wood itself because it is not a perfect surface. Most raw or natural wood has a textured surface along with knots and holes, dents, dings and scratches. Of this means that the polyurethane will find all of these surfaces and if you want a perfect top coat you will not get it. In order to get that perfect top coat you need to make the wood as perfect as you can. If you are building something from scratch then machine the wood so that it is smooth. You can then joint it and glue it. You then sand the wood down to 320 for hardwoods, pine and other softer woods to 220. Never be too aggressive with the sanding, go easy and light using a random orbital sander.
Sanding wood is important to a perfect top coat of polyurethane but you also need to seal the wood to seal the pores of it. You do this by raising the wood grain and then taking it down. Use thin shellac; quarter pound will do nicely. The alcohol in the shellac raises the grain. Use then orbital sander on its lowest setting with 320 grit paper. Let the shellac dry then repeat the process. If you are going to be staining the wood then you could use just alcohol (no shellac). Orange shellac will give many woods a dark, rich color but you can use any color you like or a de-waxed variety.
Applying the Polyurethane
This is the bread and butter of the project. The top coat of polyurethane should not be thick, clumpy or streaky. The perfect surface you want will not be achieved if the polyurethane top coat is either of these things. Use a paintbrush with soft, natural bristles and apply the polyurethane in a thin coat. Do not use a roller as they can texturize the polyurethane. Let this layer dry then sand it down with a sanding block. Pay special attention to any bumps caused by dust. Run your hand over the dry surface to feel them. Now you can turn to steel wool starting with #1 then moving to #000. Utilize a tack rag to take off dust that the steel wool created (as well as bits of steel wool). Repeat this process a few more times then use "black" 1500 or 2000 grit sand paper on a flat sanding block to wet sand the polyurethane top coat gently.