Sanding is probably the most important part of preparing a surface to be painted. It removes grease and imperfections, gives you a flat, smooth surface to work with, and creates a key that your paint can cling to. Not sanding means you’ve got a bumpy, potentially dirty surface, with no reliable key. It often leads to patchy paint, cracking, and peeling.
However, there are many different kinds of sandpaper, and you might not be sure what kind to use to prepare a surface for painting. Very coarse sandpaper could ruin your surface, and very smooth sandpaper may not remove enough dirt – so how do you pick the right grit for your project every time?
What grit sandpaper should you use to prep for paint? There are many different kinds of sandpaper, and which one you need will depend on the surface you are sanding – but most painting jobs will require you to use a high-grit sandpaper before you add the paint. Low-grit sandpapers tend to be necessary for removing tough stains, layers of debris, old paint, varnish, and rust.
What Grit Sandpaper Do You Need?
You have to choose your sandpaper grit based on your project. If you are only polishing up some wood, ready for a final coat of paint, you’ll need a high-grit sandpaper. If you’re preparing some hardwood, trying to get rid of rust, or dealing with other tough preparation work, you’ll need a low-grit sandpaper, followed by a medium or high-grit option.
Choosing the right sandpaper for your project is key to making sure it goes smoothly and you can work efficiently. If you use sandpaper with a grit that’s too low, you risk making scratches and marks on your painting surface. Low-grit sandpapers are very rough, and will deeply sand the top of your project.
If you use sandpaper with a grit that’s too high, you won’t be able to effectively tackle rust and other stubborn debris, and you’ll spend a great deal longer sanding the surface than you need to. High-grit sandpapers are very fine, and will only remove a little of the surface you are sanding at a time.
By choosing the correct grit, or working your way through the grits from low to high, you’ll make your sanding process faster and more efficient, and you’ll improve the appearance of your finished project. With that in mind, let’s look at how you determine what grit your sandpaper should be.
When Should You Use a Low-Grit Sandpaper?
You should use a low-grit sandpaper if you are working with hardwood, metal, or other tough surfaces. It is durable enough to reshape wood and take off stubborn debris or rust. #60 to #80 is very coarse sandpaper, and will quickly sand off a lot of your surface.
Most projects start with a fairly low-grit sandpaper and move on to a higher one. If the surface you’re sanding is already pretty smooth, you won’t need the low-grit, but if you’ve got to cut through tough dirt, you will.
Don’t use a low-grit sandpaper on thin veneers, polished woods, corners that you don’t want to sand away, or very soft woods. Low-grit sandpapers will cut through veneer, ruin polish, take off corners, and tear into softwood.
When Should You Use a High-Grit Sandpaper?
You will generally use a high-grit sandpaper before painting anything, even if you start with a low-grit option. The high-grit sandpaper is much finer and will smooth out scratches and bumps in your project. There are lots of grades of fine sandpaper, starting at around #180. In general, the finest you will see is around #1200.
The finer your sandpaper is, the smoother it will make the surface. This will provide the paint with an excellent area to stick to, because it creates a key that the paint can soak into and bond with. This makes it much easier for the paint to adhere properly, and it will ensure that your paint job lasts for as long as possible.
In many cases, you’ll start with a low-grit sandpaper first to get rid of dirt and old paint, and then progress to a high-grit option to smooth the surface and ensure the paint forms a nice finish. A lot of projects require you to use multiple grades of sandpaper.
However, you do need to bear in mind that different kinds of wood respond differently to sandpaper (as do other materials), so think about your material’s hardness before selecting your sandpaper. Don’t use a low-grit sandpaper on softwood unless you need to remove a layer of varnish, paint, etc., to get it back to wood.
What About Medium-Grit Sandpaper?
Medium-grit sandpaper is a good option when you want some good sanding power but you’re working with a softer material that might be scratched deeply by low-grit sandpaper. Medium-grit tends to be around #100 to #150 and it’s ideal for sanding unfinished wood, getting rid of stains, and general preparation work.
Some people start with a medium-grit sandpaper so that they can see whether they need to scale up or down. If you begin with it and find that it’s not making an impression on the surface you’re sanding, you can simply swap to a lower-grit option. If you find it’s making deeper scratches than you expected, swap to a higher-grit option.
Medium-grit sandpapers are generally best for use on softwoods, so bear this in mind if you’re handling something like poplar or pine. They won’t tear the wood up or leave deep scratches, and you can move on to a fine-grit sandpaper for the next stage.
Medium-grit will not be sufficient if you’re sanding metal or hardwood surfaces to prepare them for painting.
The sandpaper grit you will need to prepare a surface for paint depends heavily on what the surface is, but you’ll almost always want to finish with high-grit paper. If you’re sanding hardwood or metal, start with low-grit and work up. If you’re sanding softer materials, start on medium-grit. In some cases, you might begin with a high-grit and move to an even finer one to finish.