Surface Prep: Contract Cabinet Doors 10/11/2012 12:08:07 PM
So you have selected your wood species for color and evenness, manufactured your project, and now you are ready for the final sanding operation before finishing. Finish sanding is the subject of more than one blog so we will take it one step at a time.
Let’s start with one of the more common scenarios: contract cabinet doors. In my travels throughout the country, I see many shops both large and small using the practice of building the case goods for their project and purchasing the doors and drawer fronts from a vendor that specializes in making those items. There are pluses and minuses to doing this, all of which are the subject of a separate discussion.
After speaking with a few of the more popular door companies I have learned something very important about their sanding practices. When asked, “what is the final grit you sand to” most tell you 180 grit, an appropriate grit for most finishing scenarios. What they don’t tell you is the method of sanding they use and what effect it has on finishing.
Most of the larger door manufacturers, due to the large volume of sanding they do, use automated sanding machines that have a series of sanding blades stacked together to sand the surface of the wood. These blades have a series of sandpaper strips or fingers attached to them to gently sand the surface without removing any profiles from moldings or edges. These blades rotate at high speeds to work quickly and increase throughput for the door manufacturer.
Accepting the manufacturer’s final sanding as gospel can create some finishing issues that can be disastrous. Take a door that was sanded to180 grit by the manufacturer. Flip it over and resand the back of the door with 180 grit sandpaper (that’s right, the same grit) using a random orbit sander or by hand with a sanding block. Now, rub your hand over the surface and compare to a door that was sanded by the manufacturer. You will find that their door feels noticeably smoother than yours. In many cases, this is not a good thing.
Their sanding practice actually polishes the wood, making it much too smooth for today’s higher solids finishes. In the case of diffuse porous woods such as maple, this degree of sanding proves to be too smooth for the coating to adhere to properly. Add a layer of oil-based stain and the adhesion gets worse.
Far too many times I have seen a woodworker take a finished door or drawer front and drill for the hinges or handles only to have the finish pop off and start to peel before his eyes. In ring porous woods such as oak or ash this becomes less of an issue. The deep pores of the early wood give the coating something to hold on to.
Resand or "Pop the Grain"
Experienced woodworkers have learned that they must resand the wood, many times with the same grit sandpaper that was used by the manufacturer, to get more “tooth” on the surface of diffuse porous woods and increase adhesion. Although this solution will work, it is very time consuming. Consider a faster process called “popping the grain.”
In this process, a solution of one-part water mixed with one-part denatured alcohol can be sprayed on the doors. Spray one even, light coat and let dry overnight. This adds just enough moisture to the surface to swell the fibers of the wood without affecting the overall moisture content of the wood. In places with high humidity this solution can be reduced to two-parts alcohol and one-part water. This technique saves a lot of time and provides adequate tooth for proper adhesion.
It is important to note that, when staining, this affect will affect the final color of the stain and should be considered when making color samples for the project. Doing several tests on separate pieces is absolutely required before starting your first project. Try this technique on some old doors and then give it a go. You won’t be disappointed.
Until next time, keep finishing.