THE SECRET TO SILKY SMOOTH FINISHES
Have you ever noticed that whenever you show someone something you've made out of wood—whether it's a small box or a full-sized piece of furniture—the first thing they do is touch it? They run their hands over it and feel its surfaces. It’s the way humans are wired; we look with our hands as much as with our eyes.
After spending so much time on a project, pouring your heart, soul, and even your wallet into it, the last thing you want is for someone’s first impression to be spoiled by a rough, bumpy finish. You want those surfaces to be silky-smooth. And that means knowing how to properly sand your project, including between coats of finish.
That's what I'm focusing on today—demystifying what needs to happen between coats so every wood finish will come out looking and feeling perfect.
What You'll Learn
You can apply what you'll learn here to any clear film finish such as polyurethane, lacquer, or shellac—whether you brush it on, wipe it on, or spray it on.
I'm going to assume you have already sanded your bare wood surfaces smooth. If you need help with that, I have another tutorial that covers everything you need to know. I also have tutorials on how to apply the finish, which you can find in the links below this post. This lesson starts after you've put on the first coat and find that the surface, which was once nice and smooth, is now rough again.
Why Your First Coat is Rough
Two things likely happened: a little bit of dust from the air settled on the wet finish, and the wood fibers themselves have soaked up that finish and become swollen. You can reduce the effects of swollen fibers by raising the grain before you apply that first coat—this is covered in my other sanding tutorial. However, it may still have occurred to some extent, especially if you applied a water-based finish.
So, you’ll need to sand those fibers back down. I recommend around 320-grit sandpaper for this first coat. Do it by hand with light pressure. You don’t want to sand the entire coat off, but depending on how swollen the grain was, you may end up turning more of this first coat of finish into dust than you will with the subsequent coats.
By the way, if your sandpaper is gumming up, it’s likely because the finish needs more time to dry. You should get fine, white dust on the surface—not little bits of partially dry finish clogging up your grit.
Sanding Tips for a Smooth Surface
Sanding with two hands allows you to feel for imperfections to sand away.
I always like to sand with two hands—one moving the paper across the surface, the other feeling for imperfections that need to be sanded away. Again, use light pressure but be thorough. This first coat will set the foundation for all the rest, so you want it to be perfectly smooth.
When you're finished, clean off all the dust. Don't blow it off with compressed air, as that will just make the dust settle back down on your next wet coat. A vacuum works nicely, followed by a wipe-down with a little mineral spirits, denatured alcohol, or even water. Just don’t get it too wet since there isn’t much finish on there yet, and you don’t want water to soak into the grain and raise the fibers again.
Dealing with Dust Nibs and Shrinkage
After the second coat of finish is dry, you shouldn’t have to deal with the fibers as much. Instead, dust from the air that’s settled on the wet finish and dried there will be your biggest concern. Unfortunately, dust nibs can’t be avoided. I have a separate, theoretically dust-free finishing room in my shop, and I still get dust nibs. You just have to deal with them by sanding them away.
To prevent sanding off all the finish you're putting on, switch to a finer grit—maybe around 600. The process is the same: work by hand, following the direction of the wood grain as much as possible, and use very light pressure while feeling the surface to find areas that need attention.
After the second coat, I start applying all subsequent coats of finish very lightly. The thinner the coats, the quicker they will dry, and the less time there will be for dust to settle and stick on the surface.
Shrinkage and Sanding with Finer Grit
Now, dust nibs are less of a concern, but another phenomenon can pop up. All finishes shrink as they dry. Some shrink more than others, but they all do it. As the finish shrinks, it puckers on the surface of the wood. You probably can’t see it, but you can definitely feel it. The solution is the same: very light sanding with 600-grit.
The Effect of Solvents on Finishes
There’s one last factor you probably haven’t considered. Many finishes, especially those that are oil- and alcohol-based, may be partially re-dissolved by their base solvent. A can of polyurethane, for example, contains poly resins and mineral spirit solvent. As a coat dries, the mineral spirits evaporate away. But the new mineral spirits in the next coat can partially dissolve and swell the previous coat.
With some finishes, like water-based poly, this isn’t a factor. With others, like shellac, it’s unavoidable, as shellac never fully cures. Oil-based polyurethane, on the other hand, does go through a chemical curing process that eventually hardens it, but this can take a long time—maybe even a month—before it becomes resistant to solvents.
This means the longer you can wait between coats, the smoother the next coat may be.
Final Coats: Patience Pays Off
Again, this is just one of four factors to consider. You can wait a month between coats and still have to sand away dust nibs or other issues we just discussed. But when you get to your final, thinnest coats, it may pay to give them as much time as possible to dry and at least partially cure. I like to wait at least 24 hours between coats, sometimes even a few days, before applying the final coat.
That final coat is usually the thinnest, and as such, you can’t use 600-grit sandpaper to sand away the last imperfections. I use at least 1000 to 1500-grit sandpaper to buff the final coat. In fact, I often use a brown paper bag, which is cheaper than high-grit sandpaper. If you’ve minimized the four causes of a rough finish up to this point, that paper should make your finish as smooth as glass.
Conclusion
Try this on your next project. It will make a huge difference in the final look and feel of your woodwork.
TUTORIALS ABOUT SANDING AND FINISHING:
Comprehensive sanding tutorial: • So, you think you know how to sand, eh?
The interesting history of sandpaper: • The history of sandpaper is really IN...
Random Orbital Sander tips: • These ROS tricks will change how you ...
Using oil-based poly: • Watch this before using polyurethane ...
Using wipe-on poly: • Don't buy wiping polyurethane! (How t...
