Sanding – Secret to Success
Understand what your finish system requires. You have to approach sanding from the perspective of the finish and not your finger. Of course, you sand to level joints and smooth the surface; however, you are also providing a uniform texture that will help take stains evenly and provide a tooth for the finish to grab on to. Selecting the correct grit and type of paper is crucial. Many of the high solids finishes require less white wood sanding than their older counterparts. Over-sanding is not only counterproductive, but it will ultimately encourage adhesion problems.
To reduce clogging, wood should be sanded with an open coat paper; finishes sand best with a stearated paper. Wood sands are best with aluminum oxide paper since the particles fracture during use and constantly expose sharp new edges. For finishes, silicon carbide paper is the choice when sanding finishes by hand; a high-quality aluminum oxide paper works best when machine sanding a finish. Abranet discs are excellent for sanding sealers and primers as they don’t clog under these high load situations.
Micron-graded sandpaper has the most uniform grit size and makes it the best choice when machine sanding finishes or solid surface materials.
Abralon pads are used wet for repairing or polishing a finish. Mirlon pads work well for scuff sanding wash coats and toner coats. Neither of these two products will level a finish, for that you need a cloth or paper-backed abrasive.
Sanded with 120 | Sanded with 220
Sanded with 120 | Sanded with 220
When sanding finishes, be sure to let the finish dry sufficiently before sanding, but don’t wait too long. The process of spraying a coat of finish, sanding it, and then applying the next coat should optimally take place within a 24 hour period. This is very important when using any type of catalyzed finish since they do not melt into the previous coat. All catalyzed finishes bond to the previous layer by a mechanical link; they need a rough surface to adhere to, so sanding between coats is essential. The goal is to sand and apply the next coat before the previous coat of finish is totally shrunk; this way the new coat is deposited in the sanding scratches of the first coat which will then shrink around the new coat and give you maximum inter-coat adhesion.
How coarse of grit we need to sand with depends on how thick of a film we applied, how smooth and uniform the film is, and how old the film is. Sand with only as course a grit as necessary to level the surface. Remember fresh finishes will continue to shrink back after you sand them, cured finishes won’t. Sand a fresh finish with a 220 grit and you can recoat with no problem. If you have to sand a cured finish with 220 your sanding scratches might telegraph thru the next coat because the finish will no longer shrink back around the scratch. In this situation, it's best to follow the 220 with a 320 grit paper.
In sanding, we try to scratch an “even texture” onto the wood. This will help us obtain an even color when we stain. Your sander or hand block should have a soft pad. A hard pad may cut faster, but a soft pad will compensate for minor surface irregularities and will allow a more consistent contact of the grit with the surface it is sanding.
- If you sand by hand, you should duplicate your style and movements consistently from piece to piece, edge to edge.
- If you spot sand by hand use at least one grit finer than the last grit you used on the machine
- When using a sander, let the weight of the machine do the work, this will do much to ensure a consistent depth of scratch from piece to piece.
- The particles on your sandpaper wear down or break off as they are used. A worn-out 120 grit belt will polish a board smoother than a 400 grit belt. By replacing sandpaper or belts before they get too worn you will maintain a more consistent scratch pattern and depth.
Hardwood Lumber and Veneer
- Sand bare wood with an open coat aluminum oxide paper, silicon carbide is too sharp.
- Sand lumber is no higher than 150 grit.
- Sand veneers one grit higher than lumber; but no higher than a 180 grit.
- Use a progression of grits. Skipping a grit may cause uneven staining or even scratches to telegraph thru your topcoats.
- Break edges. The finished film shrinks as it dries. Sharp edges will rip the film and allow water and oil to eventually get in. Think of it as stretching a piece of Plastic Wrap over a knife blade.
- The day that you sand your wood is the day that you should stain and seal it.
- MDF faces should be sanded with 320 paper.
- MDF edges and profiles should be sanded with only 400 and 600 grit sandpaper. A course paper tends to pull out the wood fibers from the binder in the MDF, resulting in more pits in the surface. Use a course paper only when necessary.
Sanding Between Coats of Finish
- Pieces must be recoated within 8 hours of being sanded.
- To sand between coats of finish using a 220-320 stearated paper.
- Use a silicone carbide paper if sanding by hand, a high-quality aluminum oxide paper-like Royal Micro if using a machine.
- Mirlon pads, usually the maroon, or 320 paper are good for scuffing toner and wash coats. Mirlon scuffs, but it won’t level.
- Avoid using Steel Wool as it may contain oil and the fibers are hard to clean up.
- Avoid burn thru’s. Give a burn thru extra drying time before recoating. This will reduce the chance of wrinkling.
- If you are going to glaze, remember that the glaze will stick in your sanding scratches so sand carefully.
- When hand sanding, to avoid deep localized scratches use at least one grit finer than the last grit you used on the machine.
- Always remove sanding dust before recoating. Take extra care to get it out of the pores of woods like oak.
A word about swirl marks. If you have them, then there is a problem with your sander, your system, or the person using it. A few simple precautions and you can all but eliminate them from your life.
- Start with a good quality, adequately powered, and maintained machine.
- If you are using pneumatic sanders, check your plumbing. Be aware that the length and size of piping and hose will affect performance. A common culprit is the use of reducers to go in and out of improperly sized filters or regulators. Start at your compressor and look your way to the sander.
- Check your equipment. Inspect, Clean, Lubricate, Repair, Replace. Nothing lasts forever.
- Check pads and bearings. Flat pads and smooth bearings; it’s a good thing.
- Check your technique. Let the tool do the work. If you slow it down, it will swirl.
- Check your sanding schedule. Don’t make big jumps between grits. Skip steps and it will swirl.
- Check your paper. Keep it sharp. Micron-graded paper will swirl less than CAMI or P graded papers.
- Check the moisture content of your wood. If it’s wet, it will swirl.
- Use your instincts. If it doesn't look, feel or sound right, it will swirl