Sanding: a Primer
Once in a while, a customer is a woodworker too, and it happens that I get questions from them about technique. The other day, a client texted about scratch marks from sanding that showed up in his project, and he wondered how to avoid that problem. Much of this post came out of my answer to him and the conversation we had following.
Sanding is a fundamental challenge in woodworking, and it’s not an easy thing to master. I’ve been doing it for ten years, and I think I’m getting better at it. At its most basic level, sanding is scratching the surface with progressively finer patterns of scratches, until you can’t see them anymore. You start with a coarse paper that can remove whatever tool marks, stains, surface gunk, and other stuff you need to remove. This coarse paper will probably leave easily noticeable scratches in the wood. Next you use a slightly finer paper to remove those scratches, and a slightly finer paper to remove those scratches, until you’re done.
Spend a lot of time with the first (roughest) grit, being sure to get all the machine or hand tool marks out. Spend a little more time. Be patient here, and be sure you’ve gotten them all. A raking light across the surface can help to see the marks. Then go to the next grit, and work your way up through the numbers. After the first grit, the others don’t need as much time, but be sure you’re done with each before moving on to the next. That means being sure you can’t see any of the scratches from the previous before you move on to the next. Don’t skip grits. Use all the numbers you can: 80, 100, 120, 150, 180, 220, 280, 320, etc. It’s faster than skipping grits, and you’ll get better results.
Know where to start. 120 isn’t going to take out sawmill marks unless you spend three weeks sanding. 80 might, and 60 will, but it’s very aggressive, and it might make your flat surface into a potato chip. If you’re sanding hardwood veneer, such as on plywood, never use any paper coarser than 120. I usually don’t use coarser than 150. Sanding through the veneer is a major bummer. Be especially careful with veneers at the corners and edges of panels.
Know where to stop. Stain won’t absorb properly into a surface that’s sanded finer than about 150. Clear finishes generally allow finer sanding. If you have to stop at 150 for stain, a random-orbit sander’s swirl marks will be visible when you stain the piece. Follow up the sander with careful hand sanding, with 150 paper on a block of wood or cork, using long, straight strokes with the grain. The sandpaper’s marks will be hidden by the lines of the grain, mostly.
A random-orbit sander is different from a palm sander. A palm sander has a square base, and it makes little orbits. A random orbit sander has a round base, and it makes two movements: little orbits, and rotations (spinning). In combination, these two movements create a less uniform pattern of scratches, which your eye and brain won’t detect as easily. Random orbit sanders (ROS) are better than palm sanders in most applications. The difference between a good ROS and a lesser one is that the lesser ones tend to spin a lot. This makes larger, more pronounced swirls, and they’re hard to get rid of. Your sander should make little orbits (not to be confused with rotations), usually 3/16″, and under its own weight it will rotate some. You should be able to stop this rotating easily with your hand, and without fear of hurting yourself. When you’re sanding a surface, the sander should not spin fast like an angle grinder. If it does, take it apart and clean it. If it still spins fast, replace it with a better one.
Let the sander do the work. Don’t put downward pressure on it; you’ll only tire yourself out faster and wear out more tools and supplies. And try to keep it flat! Resist the temptation to lift it up and dig in with the edge of the sanding disc. Doing so will very quickly make the foam backing pad convex, which means that most of the time you’ll only be contacting your flat surface with a small part of the disc. Therefore the work will take much longer.
Don’t be cheap about sandpaper. I’ve wasted a lot of time trying to get a job done with worn-out sandpaper, and it’s stupid. The stuff is not expensive, and your time is valuable. In addition, worn-out paper can sometimes lead to stray scratch marks that can get surprisingly deep. I don’t know exactly why, but it happens.
Get to know the magic of card scrapers (aka cabinet scrapers). Before we had reliable sandpaper (which was not long ago), how did woodworkers make surfaces smooth? With good hand tools, especially scrapers, that’s how. I find sanding with power tools to be faster and more reliable, but while I’m sanding, I always have a card scraper handy to catch bits I missed in earlier stages.