Sanding. Everyone knows it’s an important part of the woodworking process, but it’s so hard to master. Sand too much, and your wood won’t stain. Sand too little, and there are scratches all over your wood. So what’s the right amount?
The best way to know when you’re done sanding is to scribble a light pencil line across your wood before you start. Once the line is gone, move up to the next grit. Repeat up to the highest grit sandpaper, then wet your wood with mineral spirits to confirm there are no remaining marks.
This is the basic method, but lets get all the details!
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A Bit of Sanding Theory
So first off – why do we sand?
We sand the wood to get all the marks off from the manufacturing process. I know what you’re thinking; you don’t see any marks.
But they’re there, and they’ll pop out the minute you put finish on the piece (see this table, where I learned that the hard way.) So we always sand before staining and finishing.
If you’re working with newly purchased wood, I recommend starting with 80 grit sandpaper, and working up from there.
The 80 grit sandpaper removes more wood than higher grits, and therefore eliminates the manufacturing marks faster than a higher grit would. It will, however, leave decent-sized scratches in the wood.
Because of that, you should then work your way up in progressively higher grits. Each higher grit will remove the scratches from the lower grit sandpaper, and leave smaller scratches behind.
In an ideal world, that’d be 80-100-120-150-180 grit, in that order.
I don’t know about you, but I rarely have all of those in stock. I usually end up doing something more like 80-120-180, and that works well enough.
Most of the time, I stop at 180 grit. The higher you sand, the finer the sawdust created. Eventually, that sawdust starts to clog the wood pores, which prevents the wood from absorbing stain. Hence, stopping at 180.
Sometimes I’ll sand a bit higher, maybe to 240 grit, if I’m trying to prevent stain absorption, like when I’m trying to stain the end grain to match the rest of the wood (see post on that here.) But for the most part, I stop at 180.
And if you’re working with wood that’s not newly purchased – maybe it’s a furniture makeover, or repurposed wood that’s been stained and/or finished, you’ll simply start at a lower grit. I usually start at 40 grit, and sand with that until all the stain and/or finish is gone, then continue forward as normal (40-60-80…)
A Quick Note About Buying Sandpaper
If you have an orbital sander (which I highly recommend,) Amazon sells packs of velcro sandpaper significantly cheaper than any home improvement store.
The selection available is different every time I purchase, so I don’t have a specific brand to recommend, but I’ve never received anything that was bad. This was what I purchased last time, but don’t be shocked if it’s no longer in stock.
Frankly, the price difference is enormous, so if you’re worried the sandpaper won’t last as long, don’t be. At $12-$16 for a pack of 75, versus Home Depot’s $8 for a pack of 7, the online sandpaper would have to be total junk for it not to be a better deal.
And it’s not. I’ve been pretty pleased with all the packs I’ve received.
How to Know When to Stop Sanding
So now that you know which grits to use (and where to start,) how long do you have to spend at each grit?
For the first grit on salvaged wood where stain and finish needs to be removed, sand until all of that is gone.
And if you purchased particularly rough wood, like furring strips, sand with your first grit until all the obvious flaws have disappeared.
But for all other grits, follow this process.
Step 1: Scribble a Pencil Line
Grab a pencil, and scribble a light line across your wood. Make sure it spans the whole width of your wood, so that you don’t miss any spots.
It’s important that this is a light line. If you press too hard on soft wood, you’ll dent the wood with your line mark, which will make it much more difficult to remove. This will result in a lot of extra unnecessary sanding on your part (although it won’t actually cause any problems, given you’re sanding with the correct grits.)
It’s important to use a pencil for this, not a pen or a marker. Pencil writes on top of the wood, while any sort of ink will sink into the pores and be more difficult to eliminate, leading to extra sanding.
And if you were wondering about crayons, I see two issues. 1) The wax might come off the wood too easily, leading to you not sanding enough, and 2) the wax will clog your sandpaper, shortening its lifespan.
Step 2: Sand the Wood
Then sand your wood with the proper grit. Sand until the line has disappeared.
Step 3: Repeat
Once the line is completely gone, move up to the next grit and repeat the process.
Be sure to clean off your wood between grits, otherwise the larger sawdust from the previous grit will remain and create new (bigger) scratches in the wood. This negates all your hard sanding work, so be sure to clean off your piece!
Check Your Work
So, given that you stop at 180 grit sandpaper, you don’t need to worry about over-sanding. The main concern then, is that you sanded enough. Here’s how you can double check that.
If you’re planning to stain/finish with an oil-based product, grab some mineral spirits. If you don’t have mineral spirits, wood conditioner works for this too. (And, per this test, it doesn’t do a whole lot to the wood if left to fully dry.)
If your stain/finish is water-based, use water. Be aware that water raises the grain of the wood, so you’ll want to do a quick pass with 180 grit sandpaper to bring it back down before you stain/finish. (This is called “dewhiskering” and is a good practice to get into before using water-based products regardless.)
Wet the wood with either the mineral spirits or water. This will highlight any marks that were otherwise invisible or hard to see. See photo below for an example.
Be sure to inspect the wood from different angles. The liquid makes the marks much more visible, but sometimes you still have to look for them!
If you still see visible manufacturing marks, wait for the wood to fully dry, then restart at 80 grit sandpaper.
Side note: Your best bet is actually perform this test twice; once after 80 grit so you can make sure you’ve remove the manufacturing marks, and once after you’ve finished the highest grit to make sure you’ve removed the sanding marks. But that means you have to wait for the wood to dry in between, which is annoying.