22

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.

12 Answers 12

22

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.)

  • 2
    Same here, 220 or 320, whatever is handy. I also use #000 Steel Wool at the end, rather than a 600 sandpaper. Sandpaper often gums up in places where a small area hasn't fully cured. I also feel like I can follow the grain a bit better, since I have to go slowly. – Steve Jackson Feb 5 '11 at 19:22
  • 1
    220, 221 whatever it takes :) Seriously though , I use 220 and a GOOD china bristle brush. – BrianK Feb 6 '11 at 3:45
  • Depending on the shine and quality of the furniture piece, I've gone all the way up to 800 and beyond. #000 Steel Wool is great and Steve Jackson suggests. Be careful that you buy good quality steel wool though as some can leave metal particles which can stain the wood. – Carl Jun 27 '11 at 10:31
  • Yup, you want some roughness for that next coat to stick to. First coat or two, you're just filling holes in the wood and leveling places where you sanded wrong. You want that final smooth coat to adhere well, or you'll get chips and flakes and stuff, long term. – Wayfaring Stranger May 8 '18 at 20:32
  • I fully agree with using a quality bristle brush, I use 400 after first coat unless it was a bad base then maybe 220, but after the first coats I move up to 600 and even 1200 , this is more for high end inside finishes. – Ed Beal May 8 '18 at 20:41
24

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:

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

  1. Sand work piece surface to 220.
  2. Vacuum work piece surface.
  3. 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.
  4. Wipe down the work piece with a tack cloth and immediately begin applying polyurethane.
  5. 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.)
  6. After drying, sand first coat lightly with 220 paper on a sanding block.
  7. Wipe off polyurethane dust with a soft cloth, then wipe with a tack cloth, and begin applying second coat.
  8. 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.
  9. 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.

  • In my instructions above, I assumed the work piece was natural wood. If stain had been applied previously, omit the mineral spirits treatment. – Max Bucks Feb 7 '11 at 7:40
4

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.

4

Lots of good answers here. I was a finisher in a cabinet shop for many years and this is how used to do it.

Avoid anything with silicone to get on your hands or near your wood project. It causes fish eye dimple defects in your clear coat and will ruin the finish. Such items with silicone include lubricants, water repellent sprays, etc. Wash your hands before starting.

Any sanding, scraping, or wiping of stain should be done going in the direction of the grain, (never across it.)

Wear gloves when using these products and in a well ventilated area.

Naptha can be used on a white cotton rag to wipe the surface to clean of any dirt and scuff marks off and also will show any white glue that was left from manufacture. White glue that is not removed will not accept stain and should be removed at this point. A glue scraper is normally used as it doesn’t sand off easily.

For new raw wood sand with the grain using 150 grit paper. If you drip sweat on the wood or it gets wet somehow, let it dry overnight, then resand. Staining over a wet spot will show as a dark spot, not pretty.

Dust it off and apply Minwax oil stain, whatever color you like, be sure your rag is plenty wet saturated with stain when you apply it with the grain, better too wet than too dry. Then wipe off excess stain with cotton rag with the grain. Let dry overnight, it must have time to dry.

I recommend the wood be lower than room temperature for workability time, that the poly doesn’t start to setup too fast while your applying it. Also dont bring the wood from a cold place to a warm place to apply the poly because air bubbles will form under the wet poly. So, let the wood acclimate to a temp of about 65 degrees. Avoid fans blowing on the piece. Also your can of poly should be about 65 degrees.

Fast dry polyurethane will dry and be able to sand after about 8 hours. Its stinks Regular poly doesn’t stink as bad but it takes a day or two to be dry enough to sand.

Ok, so the stain has dried overnight and hopefully it looks good. Next wipe any dust off of it with a clean cotton rag. Surface should be dust free. You can use a tack rag or if your hands are dry and clean just wipe any grit you feel off with your bare hand, you will feel the surface get smoother after doing this.

A soft china bristled brush is best. Stir the poly with a new stir stick slowly, so as not to get air bubbles it it. There may be flattner paste in the bottom of the can unless it is gloss, which needs to be stired until its gone.

Apply a thin coat of poly working from the top of the piece down. Let dry overnight. Test with your finger nail on an inconspicuous spot wether or not is is dry. If it still feels tacky you will have to wait until it is dry before sanding. When it has hardened, sand with 320 just to slick it off. Too much sanding on this thin sealer coat and you will burn through into the stain, so just slick it off lightly so that it is smooth and no more. Most likely you’ll burn through some of the corners from not keeping the paper off of them, in which case you can touch them up with a crayon before applying the second coat of poly.

Next dust it off with a clean cotton rag and blow it off with compressed air if you have that, or vacuum. Wipe with tack rag or bare hands until smooth. Apply another thin coat. Its done. If its a table top let it cure a couple of days before setting anything on it.

This process was for a new wood project.

To refurbish a piece, it would need to be stripped of the old finish and cleaned off with laquer thinner, (very stinky and highly flamable.) propor ventilation, respirator a must.

  • Hello, and welcome to Home Improvement. Awesome, detailed answer: keep 'em coming! – Daniel Griscom Jan 2 at 14:49
2

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.

2

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.

0

In my case it depends on the type of finish I want to achieve: heavy or light, or somewhere in between. I usually thin the urethane to get a lighter coated finish; and I always use sponge brushes to get the smoothest results. On heavier finishes I use heavier sandpaper; lighter finishes, lighter sandpaper. It mostly depends on how smooth each application turns out: higher ridges or bubbles require lower grit sandpaper. Otherwise, I follow Max Bucks detailed procedure fairly closely.

0

I just finished my first big stain and poly job, and I didn't like all the scratches that the 220 sandpaper was creating on each layer of polyurethane. Instead of sandpaper I tried cardboard, and it worked great! Got rid of bumps, created a smooth surface, and no scratches.

0

To get a perfect finish sand with 120 grit then 180. Apply whatever stain,varnish, you wish leave till properly dry then final sand with Lubrisil 400 grit then smooth with a damp white spirit cloth.( Damp NOT WET) Re apply stain/varnish lubrisil again wipe with spirit cloth and leave to dry finally buff with a soft rag.

0

I agree 80 is just way too coarse, maybe between the 1st and 2nd coat if you did a super thick layer of poly but you should try to avoid that all together by applying thin coats on each layer.

-1

I use 320 after first coat, then go to 400 for the remaining coats. I find 220 too rough myself.

-4

80 grit will work just fine in removing any dust particles in between coats.

  • 3
    Hello, and welcome to Stack Exchange. 80 grit seems pretty coarse for this purpose. – Daniel Griscom Jan 5 '17 at 19:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.