Yes, and no.
There are two different methods of sanding to talk about. One is simply using a dry sander on a piece of lumber that might have been left out in the rain or has a high moisture content.
The other, known as wet sanding, it a method that’s used to refine a finish on pieces of furniture. Let’s learn about both of them, and the benefits behind each.
You’ll use one method in general construction where aesthetics are not yet part of the project, and another to get that fine detail and smooth finish for furniture and in-home projects.
What is Dry Sanding?
Dry sanding is what everyone pictures: a piece of dried lumber that’s sparking off a bunch of sawdust as an electric sander glides across it.
This is sanding dry wood for it to meet the right size needed for the job, while also trimming it in certain areas for hinges and other similar things.
When the wood moisture content is below 9%, dry sanding breaks down the top layer of the wood (usable material percentage) to reveal the softer layer underneath.
Wood has a ton of natural fibers that are aroused when you sand it, which is where wet sanding will come into play.
You can smooth edges, remove excess wood and shrink the overall size of whatever you’re working on.
This is the rough stage; you can’t get a lacquer-smooth grit to the wood, but it will prepare you to use wet sanding to make that happen.
You will traditionally use grit ratings of 100 to 180, and very rarely higher than that during dry sanding. Higher grit ratings are almost exclusively used in wet sanding.
What is Wet Sanding?
This is completely different from dry sanding.
For one, you won’t be using an electric sander for any reason. You’d basically destroy it if you even tried doing this—it’s just not a good idea.
You will find some articles online that tell you it’s okay to apply wet sandpaper sheets to your electric sander, but it’s a recipe for disaster (and may void your warranty on the sander).
Wet sanding requires you to take your sandpaper, or possibly your sanding brick, and soak it in water for twenty-four hours.
You can do a quick-soak, which is only fifteen minutes, but this is only to be done in a hurry and for lower tier jobs.
Soaking the sandpaper in water for an extended period of time ensures it’s soaked into everything completely.
Water acts as a lubricant during sanding, which will carry debris, sawdust, and any dirt that has accumulated on the surface of your project.
This results in the smoothest finish you’ll ever feel on any completed project. Auto body detailers use this method (in different fashion, but with the same principle) when finishing vehicle exteriors.
With woodworking, you’ll use fine grit sandpaper, from 200 up to 2,000.
The higher the grit, the finer the finish, though if you took dry 2,000 grit sandpaper to wood (without an electric sander) then it would feel extremely difficult to actually move it across the wood in an efficient fashion.
Water helps it move along smoothly, and eliminates all previous scratches and impurities in the surface of the wood.
Wet sanding (at least with woodworking) doesn’t include additional water, just whatever is withheld in the body of the sandpaper or sanding brick.
We aren’t trying to soak the wood. The surface can air dry fairly quickly, but if we saturate it, then the wood will expand and fibers will stand on end again.
Wet sanding is done to a piece post-finish, meaning after you’ve applied your first coat of finish to the project.
This helps collect any fibers that are out of place, stand them at attention, and then the high grit sandpaper shreds them away to leave a delightfully smooth finish.
What Happens if You Dry Sand Wet Wood?
Nothing good I’m afraid.
There’s a difference between wet wood, and wood that has been wet sanded. As mentioned before, when you wet sand, you intentionally soap the sandpaper and apply it to dry wood.
You can’t just bring a dry electric sander or a sandpaper brick/sponge to a piece of thoroughly wet wood. What happens is that the grit gets mushy, and doesn’t do its job.
The result is an uneven surface, ruined sandpaper (that is barely used, mind you), and you’re still left with destroyed wood that isn’t usable.
What is an Ideal Moisture Content?
Somewhere between 7-9%. Wood is going to have moisture; it’s a fact.
When wood is too dried out, it splits easily and chips on the ends. When it’s too wet, it gets a foul low-tide odor, and appears too soft even to the touch.
While the wet wood will be more malleable, it isn’t likely to dry well.
Tips for Wet Sanding and Letting Wood Dry
Wet sanding is different from normal sanding. Whichever method you’re planning on using, these tips and tricks will help you along your way.
1. It’s all about ventilation. Your piece(s) of wood need a couple of days to dry, especially after a wet sanding.
Regardless of the shape of your wood, you need to lay it over something that will offer a light breeze and keep air circulating.
It’s not a bad idea to get some large box fans nearby if you’re drying them in a garage or a workshop.
2. The temperature and humidity will tell you how long the wood takes to dry before you can continue sanding it.
On average, it’s going to take two to three days for the wood to properly dry (about 1/16 an inch below the surface level/workable exterior of the wood).
They should dry indoors without being pressed up against one another.
3. Use a heater to dry your wood. If you raise the heat, and therefore lower the humidity in your workshop, you’ll be able to dry your wood much faster.
Just because it feels dry on the surface doesn’t mean that the interior is dry enough to work with. Sealing wood that is still moist in the center could cause mold or wood rot to form.
If the pieces have yet to be cut, you can trim a 1.5” piece off of the end of your raw lumber and inspect the direct center to get a better idea of how dry the wood is.
4. Wet sanding can go horribly wrong if you aren’t careful. Water acts as a lubricant for extremely high grit sandpaper, so you could shave away too much of your project without even realizing it.
Visually, the water will distort what you’re looking at. It’s important to take it slow, and regularly inspect your project from different angles to ensure you’re not whittling down one side.
5. Use a pre-stain wood conditioner. This basically acts as a primer for when you put your finish on the wood, giving it a better bond to hold onto.
This isn’t necessary, but after using it a few times, you’ll be hard-pressed to work without it. It helps make the entire process go a little smoother, which helps the wet sanding go easier as well.
6. Before lumber is sent to hardware stores to be put up for sale, they can be cured for as long as six months.
Most of the moisture is taken out of the wood, but not all of it.
For woodworking projects where you’ll be using a wet sanding option, you should still dry out any new wood for one week before putting it up against your tools.
Think of this as a precautionary measure.
Can You Wet Sand Painted Wood?
You can, but it won’t do much good.
To remove paint and other layers of coloring or finish, you need to use a dry sanding method to remove all of it. After that point, you can apply a new finish and begin wet sanding.
Can You Over-Sand Wood?
The higher the grit, the easier it is to over-sand your wood. Be wary with wet sanding methods, and be certain to inspect your project often to ensure you aren’t removing too much wood.
Can You Wet Sand Varnish?
You absolutely can wet sand varnish. Wet sanding is used to add a refined, smooth feeling to your woodworking projects, and is intended to be used after the first coat of finish has been applied.
The thicker the varnish or polyurethane, the higher grit rating you will need to effectively sand it down.
Does Wet Sanding Remove All Scratches?
If you do it properly, it will remove every scratch from the surface of your project. The depth of the cut or scratch matters.
If it is too deep for wet sanding to fix, consider using a wood filler and sealing it off before wet sanding.