What grit sandpaper should I use between coats of polyurethane?

Question

I'm planning on applying polyurethane to a pine bench I've finished staining. I decided on foam brushes for application, but I wasn't sure what grit sandpaper I should use between coats. Is 400 good enough? Or should I use 600? Or both?

Also, if anyone has any recommendations for application other than foam brushes, I wouldn't mind hearing them.

Answer 1

I use 220 grit. You sand polyurethane to (a) remove any dust bumps in the lower coat, and (b) cut the surface of the lower coat so that the upper coat has more surface area to bond to. You don't need the lower coat to be super-smooth because the upper coat will cover the roughness from sanding anyway.

For application, I use a china bristle brush. I've tried foam brushes, and I just think that I get a better finish with bristle. YMMV. (I do use foam brushes for applying pre-stain and stain; I think they work better -- for me, at least -- because stain is runnier than polyurethane. Also you have to wipe the surface down afterward, so brushing isn't the last step in the process.)

Answer 2

Read the instructions on the can. See what the manufacturer recommends.

Sandpaper is fairly useless on polyurethane because the heat generated by the friction causes the polyurethane to melt and block up the grit. And the finer the grit, the faster it gets blocked up.

I have been using polyurethane for more than 30 years. Here is my routine to achieve a fine finish:

1. Sand work piece surface to 220.

2. Vacuum work piece surface.

3. If you do not have a dust-free environment, try to apply polyurethane outside on a calm day. Dust particles falling on your work are a major problem. Stay out of the sun, though.

4. Using a soft cloth, rub a very light coat of mineral spirits into the work. This will give the polyurethane a head start penetrating and “gripping” the wood and will retard the drying time slightly. This will also allow the grain to appear more vividly so you will get some idea of the finished product.

5. Wipe down the work piece with a tack cloth and immediately begin applying polyurethane.

6. Apply polyurethane with a fine brush, working across the grain first, then with the grain. Work quickly, overlapping and moving the polyurethane along the piece. Apply only a thin coat. Do not attempt to re-brush the polyurethane once it has “leveled.” (When the brush strokes disappear.)

7. After drying, sand first coat lightly with 220 paper on a sanding block.

8. Wipe off polyurethane dust with a soft cloth, then wipe with a tack cloth, and begin applying second coat.

9. I do not use sandpaper on the second coat. I use medium steel wool on the second coat, and fine steel wool on the third coat.

10. After a few weeks, you can apply a light coat of carnauba wax using very fine steel wool or just a soft cloth.

Note: Never work directly from the polyurethane can. Pour the polyurethane you need into a clean plastic cup, then work from the cup. Discard any unused polyurethane and the cup. Never pour unused polyurethane back into the can. If you do, you will just be adding more dust particles to your next finishing job.

Answer 3

I've been using polyurethane for 40 years or more. I prefer the 320 grit between coats. Try to get the "non-filling" type, like the waterproof or aluminium oxide. After the first coat, all you are trying to do is remove the bumps and scuff the surface for the next coat, so 220 is just too rough.

For the final coat, if I really want a smooth coat, I will use a spray can, just for the last coat.

I've never found any significant difference between 400 and 600 in terms of the result.

Answer 4

I build custom furniture for clients part-time. I do about 3-4 projects a year based on the time it takes for completion. The longest part of the job is the finish. I use a 0000 steel wool after 3 light coats. Followed by 2 light coats and a sanding of 400. Then one coat wet sanded with 400+. Then a final coat. Followed by the cure and turtle wax compound, polish, then wax. My carpentry is average but i bill top dollar because of the finish.

Answer 5

When using polyurethane, oil or water based, I sand with cardboard. Yes cardboard, from a box. I let the first coat cure for at minimum 3 days at optimal conditions. Just don't rush the process. Let's be honest, it took long enough to do the project in the first place so why rush the final step. If there are any runs I use 220 grit and a second light coat. Once you are happy with the finish, sand with cardboard and apply the final coat. I always spray now because the final product is always better than brushing. Grab a few cans of Varathane poly from the local hardware store and go easy. The best tip is use multiple lights and see the reflection on the surface. Take your time and good luck.