When you really want a glossy finish, you’ll need to wet sand. Wet sanding is a process that’s often used on car paint jobs, guitars, and even 3D prints to give them a mirror-smooth look.
Wet sanding is typically done after dry sanding to get an even finish. Unlike dry sanding, wet sanding is done not to shape a surface, but to remove the large scratches left by dry sanding. When done correctly, the surface will be slowly leveled out, and the scratches left behind will become smaller and smaller, until the light they reflect no longer makes them visible.
1. The wet part of wet sanding refers to the use of water or some other liquid as lubrication to help carry away grit particles that are removed. Without the liquid, material can build up in the sandpaper and leave behind scratches that are larger than the particle size, ruining your finish. In general, the best liquid to use for most materials is water with a little bit of detergent in it. (Dish soap works well.) The detergent lowers the surface tension of the water, and helps wet the paper and the material more thoroughly, reducing scuffing. If you’re sanding bare metal, you can use WD-40 as lubrication instead of water.
2. Not all sandpaper is created for wet sanding, so make sure that the sandpaper you’re using is specifically rated for it. 3M’s “Wetordry” is the standard type, and can be found at your local hardware store. You can fold the paper over on itself to create a thick piece to hold on to, but it’s even better to wrap the sandpaper around a backing pad. Alternatively, you can buy sanding sponges which have the abrasive adhered directly to the sponge. This helps the sanding surface conform to the shape of the material.
3. In order to thoroughly wet the sandpaper, many people recommend soaking it in liquid overnight. This ensures that the paper won’t absorb any more moisture during the sanding process. If you don’t have time to soak the paper overnight, aim for at least 15 minutes of immersion prior to sanding.
A wise maker once told me the key to sanding is to forget about the concept of “sanded.” Don’t get impatient! It can be boring, but it’s important to be thorough. Put on your favorite show, or listen to your favorite music, and really get into the process.
What is “grit?”
The grit of a sandpaper refers to the size of the abrasive particles bonded to the paper. The higher the grit, the smaller the particles, and the finer the scratches left behind.
The first grit that you start wet sanding with depends on the previous grit you used to shape the object, and how smooth the surface is. If you last dry sanded with 600 grit, you’ll want to choose a wet sanding grit that’s around 800–1200. In general, you’ll want to start wet sanding around 600–1200 grit, and follow the same dry sanding rules for working up through the grits, jumping up 200–500 grit between passes (depending on how meticulous you want to be). You can buy wet sandpaper up to about 3000 grit, but most people will be satisfied around 1500 or 2000 grit.
One big difference between dry sanding and wet sanding is the movement used. Dry sanding requires small circles; wet sanding uses straight lines, alternating direction between passes. This way, each successive pass works to remove the scratches from the previous one. Be sure to use a light touch as well — we’re not trying to remove a lot of material, just the scratches!