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Polyurethane is a layered finish that is applied in multiple coats. Sanding between coats promotes adhesion and a smooth finish. In this article, learn how to sand polyurethane between coats, how to prepare wood to receive polyurethane through sanding, and whether or not to sand your final coat of poly.
Sanding Wood to Prepare for Polyurethane
Sanding between coats of polyurethane is important to achieve the best possible results. However, if you apply polyurethane to an uneven surface, those imperfections will carry through to the final coat.
- Remove any old finish from the wood. Denatured alcohol will dissolve and remove shellac. Mineral spirits or acetone can be used to remove lacquer. If you’re removing and replacing a coat of varnish, you’ll likely need a chemical varnish stripper. Removing a finish that has stain under it is trickier. Or, you can also sand the old finish away.
- Choose a sanding tool. You can use a random orbital sander, which will evenly and quickly remove old finish. Sanding by hand is also effective, albeit much slower. To speed up material removal, purchase or make a sanding block. Sanding blocks distribute pressure evenly across the sandpaper, providing superior results to simply folding a piece of sandpaper in half and scrubbing away.
- For new wood only: begin sanding with a coarse-grit sandpaper. A freshly-milled board or plank needs special attention before applying polyurethane, as the surface of the wood will be very uneven compared to previously refinished wood. Attach an 80-grit sandpaper to your random orbital sander or sanding block. Sand in the direction of the grain, not against it. For refinished wood: skip to the following step
- Sand with a medium-grit sandpaper. The coarse sandpaper will remove the largest lumps and bumps in your wood, without creating deep gouges that will need to be filled in or sanded out later in the process. It will also miss some smaller imperfections, lumps, and bumps. A medium grit sandpaper, such as 120-grit, should produce a flat and even surface that is smooth to the touch.
- Sand with a fine-grit sandpaper. The previous two steps have likely left visible scratches in the surface of your wood. 220-grit sandpaper should remove any trace of the sanding and preparation process. This is especially important when spraying a clear polyurethane, as it will clearly show any scratches or gouges in the wood underneath.
- For wood with a dense grain only: Check the surface of the wood for scratches after completing the fine grit sand. Dense-grained wood readily shows scratches even from fine-grit sanding. Extra fine-grit sandpaper, such as 320 or even 400-grit, can be used to lightly sand away any scratches left behind.
- Check your progress. Shine a light horizontally across the workpiece, or hold it up to a light if it’s small enough. Take a look at eye level – the surface should look smooth and flat. Use your bare hands to stroke the surface of the wood, feeling for any imperfections, rough spots, or splintering. Then, use a tack cloth to wipe over the surface of the wood. When you can’t see or feel any imperfections, and the tack cloth glides easily over the wood, you’re done sanding.
- Clean the dust. Sanding produces a lot of dust, some of which makes it into the air. Sawdust inhalation can cause respiratory problems. You should always wear a dust mask when sanding to protect your airways. After sanding, your workplace will also be covered in sawdust. Use a vacuum to suck up the majority of the waste. A tack cloth is the final step in removing all traces of sawdust from your work. Remember that anything on the wood when the polyurethane is applied will be sealed in for the life of the finish.
How to Sand a Polyurethane Finish
Use these instructions to build a strong protective barrier with your polyurethane finish.
- Apply polyurethane to a smooth surface. If you are looking for a smooth finish, you need to start with smooth wood. See the instructions above for details about how to prepare old or new wood to accept poly. No matter what type of finish you select, preparing the wood helps produce a smooth finish.
- Wait for the polyurethane to cure. Unlike finishes like polycrylic, polyurethane contains a solvent. As the solvent dries, the polyurethane dries as well. Curing is a different process than drying, and can take longer. Consult the label of your chosen polyurethane for instructions on how long to wait between coats.
- Remove large drips with a razor. Leave drips alone until they are dry and hard. Then, take a single razor blade (the kind used in utility knives) and use it as a scraper. Gently and slowly scrape the drip off, leaving behind an even surface. Angle the razor as close to the wood as possible, to avoid removing more material than intended.
- Sand the polyurethane. Use sandpaper with a grit between 180 and 220. If you’re sanding a very large surface, such as a floor, it makes sense to use a power sander. For smaller projects, sanding lightly by hand is best as it will remove less material. Use a sanding block or foam block wrapped in sanding paper to buff out any drips or debris that dried in the poly.
- Clean the sanded surface. Tack cloth is the best way to remove all traces of sawdust from the freshly-sanded polyurethane. If you don’t have tack cloth, a lint-free rag will do.
- Repeat. After each coat of polyurethane is applied and dried, sand out the imperfections and wipe the wood clean. Three to four coats of polyurethane applied in this manner will cure to a smooth, hard, lustrous finish. When you get to the last coat, you have a few options, including sanding. Check the section below for details.
Do You Need to Sand the Final Coat of Polyurethane?
Polyurethane is usually applied in multiple layers. Sanding between each coat, as described in the steps above, removes any debris while providing the right surface for the next coat of polyurethane to adhere to. This helps to achieve a smooth and strong finish.
As the sticky polyurethane cures to a hard finish, insects or dust can become trapped on the surface of the wood. This isn’t an issue for the first several coats, because you’ll sand them away between each coat of poly. What do you do when you get to the final coat? You have a few options:
- Do nothing. If you somehow avoided catching an insect in your workpiece and you don’t mind a bit of dust and a few fibers, you can just call the project finished. We wouldn’t recommend this for a DIY tabletop or highly visible surface, but it’s probably fine for DIY shelves of a bookcase.
- Use sandpaper. Use the abrasive material to dislodge any bugs or dirt without removing any polyurethane. Gently wipe extremely fine-grit sandpaper over the affected area, using the lightest possible touch. 600-grit sandpaper should work well, but if you have a higher grit paper on hand, use that. Sanding is risky, and can end up scuffing your work. There’s a better way: buffing with steel wool.
How to Buff Polyurethane
Polyurethane shows every scrape, scuff, and scratch made by all but the finest-grain sandpaper. Buffing with steel wool and a finishing paste is a better choice.
- Gather your materials. You will need steel wool, and a finishing paste that is clear, or close to the color of your wood. 0000 steel wool is best. You can find it at most hardware stores. Finishing paste is a woodworking product, also known as ‘finishing wax’. It works by filling in any minor scratches in the surface left behind by the sandpaper, so the finish reflects light more evenly, making it appear shinier and more lustrous.
- Buff with steel wool. Put some finishing paste onto a pad formed from 0000 steel wool. Rub the paste into the wood using small, circular motions and working in the direction of the grain. The technique is similar to shining a leather shoe.
- Clean the surface. Use a clean piece of steel wool to wipe off any excess finishing paste. Wipe the workpiece with clean steel wool to remove any imperfections in the finish. Your polyurethane should now be smooth, even, and lustrous. This buffed effect can last for several years. When it starts to look dull, reapply finishing paste with a rag and renew the effect.
You’ll want to make sure that you clean your polyurethane finish frequently so that it lasts longer.
Note that if you apply a wax-based finishing paste over your polyurethane, you have created a barrier that no other finish will stick to. If you want to refinish the pieces in the future, you will first need to remove the wax. Wet a rag with mineral spirits and use it to wipe off the wax.
A clean rag can only accept so much wax before it spreads it around instead of removing it. Have a clean stack of rags on hand and change rags frequently.
Sand wood thoroughly to a smooth surface before applying your first layer of polyurethane. Sand between coats of polyurethane to build a strong, durable, and attractive finish that will protect your wood for many years. You can also lightly sand the final coat to remove debris, but be very careful not to scuff it.