I have several sanders in my shop, but I definitely use the random orbit sander the most. Out of all things to screw up in woodworking, sanding errors can be the most infuriating, because we mostly sand near the end of a project, after we’ve spent hours being so careful building and assembling. It’s like twisting an ankle at mile 25 of a marathon. Or losing a wheel in the last mile of the Indy 500. (That can happen, right? I’ve never actually watched a car race.) Plus, sanding mistakes can be very difficult to repair.
1. Cupping and rounding
In the real world, joints aren’t always as perfect as they are on every Instagram post ever. So let’s say you’ve joined two boards together that aren’t exactly flush.
It’s just a little off and a couple minutes of 60 grit will smooth it out. You fire up your sander and attack.
Yay, both sides are flush. But now you have a cupped board because you neglected the middle. And if this was the bottom of a box or something, now it’s a rocker.
There’s nothing wrong with sanding uneven joints, but just make sure you sand equally along the length of the board. Making a pencil mark along the entire board can help. Get it to disappear evenly.
By the way, whenever you need to test if something is flat, use your table saw. It’s probably the flattest surface in your shop. Then if there’s no wobble, you can confidently blame the floor.
2. Corner Rounding
This is a really common one. A sander is a simple way to ease over the sharp edges of a tabletop. If you are going to do this, pay close attention to the corners. It’s very tempting to maintain the same sanding motion and hit the corners, right?
Two things can happen. First, you’re a bit too enthusiastic and basically remove the corner. It only takes a second for a power sander to nerf that corner. Sure, it won’t poke an eye out, but rounded corners can look very odd.
Second, when you sand the vertical edge of a corner you’re left with a bulge. The main reason this happens is that there is flex to the sanding disc. When you press the sander against a corner, there is concentrated force on a small part of the sander and the disc cups. Even if you are trying to be careful and rock the sander, this can happen. Plus, the sander is in contact with so little surface, holding it completely straight is impossible.
The solution is to use something rigid like sanding stick and ease over these edges by hand.
3. Neglecting the faces
We sometimes think about the purpose of sanding as removing gouges, scratches, and any other rough spots. And while those are important, try not to think of sanding as a spot procedure. The entire project needs to be sanded in order to achieve a good finish. Treat the sanding phase of a project with as much attention as you give to making accurate cuts.
Probably the most neglected parts are wide faces, say the top of a table or the sides of a bookshelf. We might spot sand a few glaring problem areas, then just give the rest a quick once-over. These surfaces might appear nice and smooth, but often times surface imperfections won’t appear until you apply a finish.
The main purpose of sanding is to prepare all surfaces to accept a finish, so get some headphones and music and spend some quality time in the sanding zone.
4. Oversanding plywood
Everything I just said about not neglecting the faces? Here’s the exception. And it’s a BIG exception.
Most plywood you will use for a woodworking project is pre-sanded. It may still need a little sanding, but not much. Feel the surface with your hand: if it feels a little fuzzy, you can knock down some of those wood fibers. But I never sand the surface with anything more coarse than 120 grit sandpaper. And even then, I just lightly sand, being careful not to hold the sander in any one spot.
The problem is that plywood is made up of multiple layers of cheaper wood with a thin veneer of good wood on top. If you use hardwood plywoods, say cherry or walnut, that veneer is even thinner. It doesn’t take long to sand completely through this paper thin layer and ruin the surface. This is exceptionally devastating because not only is it usually on a very visible surface, but there is really no way to fix it.
Try sticking some painters’ tape to the plywood, to protect it while you are sanding. And of course, try your best to align the edge banding flush to begin with. If you have a router, you can use a flush trim bit to even out these two surfaces.
The second problem that can lead to oversanding is trying to remove dried glue. If you get any glue on the plywood, you can sand it off with 120 grit sandpaper, but just be super careful.
5. Sanding inside corners
It doesn’t take long to figure out that a round disc will not be able to sand inside corners. But what we might forget is how difficult it is for a random orbit sander to sand all the way to the edge of an inside surface. Let’s say you are sanding the bottom shelf of a bookcase…that’s the non-removable one. You want to sand right to the inside edge so you run the sander all along the surface, pressed against the perpendicular side.
What you will discover is a gouge all along that surface where the edges of the sanding disc rubbed.
If you are really prone to doing this, you can run some tape along the edge to protect it, it’s probably fine to just avoid sanding that close. You’ll have to sand the corners by hand anyway, and it will just take a little longer to hand sand those inside edges.
Plus, these inside surfaces are usually secondary and won’t be very visible. Usually a light sanding is fine. As with all sanding, pay the greatest attention to your primary surfaces…the ones people will see.