Deciphering the Sandpaper Puzzle

Sandpaper is an integral part of any almost every kind of home repair- especially finish work. Painting, carpentry, refinishing, flooring, woodworking. They all require sanding at some point in the process, so it’s imperative to know what you’re doing.

There are a least 1,387,902 different types of sandpaper (that may be a guess, but there are a lot!) There are grits as low as 8-grit (which is essentially a few rocks glued onto paper) backing all the way up to over 5,000 grit for very specialized uses like computers and jewelry.

The trick with sandpaper is finding the right progression of grits to get the finish you desire without going overboard. Most sanding jobs require at least two different grits used in succession, and I usually prefer a progression of three grits to get the right surface.

Learning what grit to start with and which to end with is the key here, so let’s walk through some of the most common jobs around an old house and what type of sandpaper should be used.

Work Through the Grits

Don’t skip too many grits. Sanding first with 24-grit paper then jumping all the way up to 80-grit paper will leave the surface rough and unattractive. Sanding damages wood (microscopically) and each grit is designed to remove some of the damage of the previous grit and hide it, so, working through the progression of grits is the best way to great results.

Is it necessary to use every grit? No, and there isn’t a grit available at every number. You don’t need to use 50,60,80 (I don’t think 70 grit even exists) You could work 50, 80, 100. Skipping one or possibly 2 grits between sanding is fine. Moral of the story: no big jumps.

Heavy Stripping or Stock Removal

This is where the big guns come out! The most aggressive sandpaper you’ll usually find without special order is probably 24-grit, which is more than enough the strip anything you need. Decades of old paint (read here about lead safe work methods), flooring adhesive and other glues, and heavy stock removal can all be taken care of very quickly with either 24, 36, or 40 grit sandpaper. Use the least aggressive paper you need that will get the job done effectively so you have less finish sanding to do later.

Preparing Old Wood for Paint

This depends on the condition of the wood you are working with. Paint does not adhere well to grey weathered wood so it’s imperative to get down to healthy wood before painting. Usually working in a progression of 60, 80, 120 is the best way to get the surface properly prepped for paint.

If it’s particularly rough, you can start at 50-grit, but anything below that and you starting removing too much wood. Note that I am stepping through the grits along the way working my way up to a finer grit.

Preparing New Wood for Paint

Believe it or not, wood fresh from the lumberyard needs to be sanded before painting. This is because of a phenomenon called “mill glaze”. A piece of wood is said to have mill glaze when the surface is extremely smooth, even to the point of being glossy due to some piece of the milling process. This results in a surface that is too slick to allow paint to adhere properly. The best treatment is a light sanding using 120 or 150-grit to remove the shiny surfaces and prepare the wood for painting.

Preparing a Previously Painted Surface

Walls, trim, baseboards, casing, cabinets and other items are often refreshed with a couple coats of paint from time to time. For matte surfaces there isn’t really any sanding that needs to occur, but for shiny surfaces like semi-gloss or high-gloss paint, a light sanding with 180 to 220-grit paper is imperative for a good paint job that won’t peel. Lightly scuff the surface until any shine is gone and you have a matte surface then you’re ready to go.

Primed Wood

Pre-primed wood is sometimes subject to mill glaze issues as well, so, following the above suggestions would be helpful. For bare wood or metal that has recently been primed, a light sanding using 150 to 220-grit paper to knock down any rough spots and any raised grain will get you a super smooth surface for your finish paint.

Preparing Wood for Stain/Varnish

Stained wood is especially prone to showing sanding marks if you aren’t careful. Work through the same 60, 80, 120-grit process I outlined above, but take extra care to make sure each progressive sanding doesn’t miss any pieces, or else they will show clear swirl and scratch marks.

You also want to avoid the tendency to sand to too fine of a grit when staining. The finer the grit you use, the less open the woods pores are and the less readily they will accept stain evenly. Anything higher than 150-grit and you will begin to have problems with stain color and penetration.

Between Coats

Between coats of varnish it’s almost always a good idea to sand. Polyurethane and varnishes create a super smooth surface that will not allow for good bonding of the next coat unless they are sanded lightly. 220-grit is the industry standard for sanding between coats of polyurethane. Just make sure to allow ample drying time before sanding. If the paint or polyurethane isn’t creating a little dust then it’s too early to sand and it needs more time to dry.

I normally don’t sand between coats of latex paint, but sometimes for high or semi-gloss oil-based paints, a light sanding at 220-grit provides good insurance against peeling.

Hand-Rubbed Finishes

Hand rubbed finishes on wood require some very fine sanding and even some wet sanding. This is where you start getting into the specialty grades of sandpaper not available at every hardware store. The process is fairly complicated, but I wanted to touch on it briefly so that you know it is an option.

Typically after the final coat of finish is cured, you would come back with a 600-grit or thereabouts sandpaper and wet sand the surface to remove any impurities or dust nibs. Then after the sanding, slurry is wiped away and the surface cleaned you would come back with 0000 steel wool and a lubricant like Behlen’s Wool Lube and polish the surface further creating a super smooth finish. Then, the finish is cleaned and coated with a couple coats of finishing paste wax.

Rubbed out finishes are not easy to do, but for high end woodworking, it really is the finest finish you can get.

Finish the Job

When it comes to painting, sanding is only one part of the process for proper surface preparation. Don’t forget the others like cleaning, priming, and painting in the right conditions to get the best paint job you can.

Hopefully, this post will get you pointed in the right direction to choosing the right sandpaper. Cleaning the dust is now up to you!