What Is A Polyurethane Varnish Finish?
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Polyurethane finish chemical structure is very similar to plastic. Once dried it provides a hard finish that is quite durable that is water resistant. However, when exposed to a hard shock it tends to fracture and flake off. The fractures will look white in color.
Common complaints about polyurethane is that it is you can’t get a bubble free finish with it, it is hard to repair and difficult to stripe off from wood furniture. There are plenty of chemical strippers available at your local home improvement store that will stripe polyurethane finish, if the directions are followed. Don’t try to take short cuts, it isn’t worth the frustration.
In actuality, polyurethane is a good choice for finishing your wood working project for the home woodworker. It is relatively inexpensive and is its own sealer. It seals and protects the stained surface.
Polyurethane finishes are divided into two different categories based on the type of ‘base’ used. The bases are oil and water. The ‘base’ holds the polyurethane and binders in suspension and prevents them from forming a solid coat. As the base evaporates, the binders and other chemicals react and produce the finished product a polyurethane finish.
You don’t need an expensive fancy brush to apply the finish. A brush whose bristles are made of plastic will give nice results. Shoot even an inexpensive foam brush will produce nice results. After a coat has been applied the brush will need to be thrown away. It is very difficult to wash all of the polyurethane varnish out of the foam. I would not recommend using a foam brush with oil based polyurethane. The oil may dissolve the foam, which will lead to a very crappy finish.
Personally, I like to use the water base polyurethane finish. It doesn’t have the strong odor that oil has, it dries fairly quickly especially on a hot dry day and it cleans up easily with water.
As with any finish, work with a small area at a time. It depends on the piece that I am finishing if it has a large flat surface then I will work with a one foot by one to two foot rectangle. Apply a thin coat to the wood, not too thick as it will cause runs in the final finish. It is always better to apply several thin coats instead of a couple of thick coats and then get runs in the final finish. Check out my free finishing tips newsletter, I have a tip on fixing runs in your dried finish.
When moving on to the next section, make sure to overlap the finish. That is to start brushing from the wet area to the dry applying the finish.
Always finish the edge first and then work your way into the middle, especially if you are working on a table top, or raised panels. By starting at the edge and pulling the brush in towards the center a fair amount of finish is scraped off at the edge and a big, fat, ugly run is created.
Finishing turned pieces such as legs and spindles, start at the top and work your way down. Apply the finish to each section or transition point. For a turned leg, I would apply the finish to the square part first and then the next turned section until I reached the bottom of the piece.
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