# How to sand between coats of polyurethane?



## HarryS (Aug 10, 2015)

So I put my first coat of semi gloss polyurethane on my pine desk that I stained with red oak last night, it has dried and I'm going to apply the second coat but wanted to know exactly how this sanding process works for polyurethane? Very few videos show this and each is slightly different but I need to make sure I do this right. I'm using 220 grit for this.

There are parts of it where it feels grainy to the touch etc so am I supposed to focus on those areas more than the rest of the board? Do I just give it a light scruff back and forth once across the boards? Or give it a (still pretty light) sanding going back and forth a couple of times across the boards the same way I did with the wood (using 220 grit) before I stained it? Whats my goal for this part of the process? Thanks!


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## Steve Neul (Sep 2, 2011)

The process of sanding between coats is just to smooth the finish where it raised the grain of the wood. If you are feeling the texture of the wood, oak for example that is normal and you shouldn't try to sand that out. If a glass like finish was desired on a open grain wood this should have been taken care of before the polyurethane was applied. A grain filler should have been used to fill the open grain. You can fill the grain with the finish however it takes a lot of coats and a lot of sanding between coats. Doing that much sanding you need to be careful not to sand through the finish but still not build an especially thick finish. 

Under normal circumstances sand the finish with the grain with 180 grit to 320 grit sandpaper until it feels smooth to the touch. If the finish is especially rough you might have to sand it a little and add another coat and sand it again before you get the finish smooth. It's just very important not to sand through the finish. This is especially easy to do on the corners of the project. The sandpaper tends to try to round the corners which tends to take the finish off very quickly. One product that is very helpful hand sanding is a Glit sanding pad. https://jackpauhl.wordpress.com/tag/glit-sanding-pads/ These foam pads are easier to hold onto than sheets of sandpaper and also tend to hold the sanding surface more flat than sheets.


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## HarryS (Aug 10, 2015)

Steve Neul said:


> The process of sanding between coats is just to smooth the finish where it raised the grain of the wood. If you are feeling the texture of the wood, oak for example that is normal and you shouldn't try to sand that out. If a glass like finish was desired on a open grain wood this should have been taken care of before the polyurethane was applied. A grain filler should have been used to fill the open grain. You can fill the grain with the finish however it takes a lot of coats and a lot of sanding between coats. Doing that much sanding you need to be careful not to sand through the finish but still not build an especially thick finish.
> 
> Under normal circumstances sand the finish with the grain with 180 grit to 320 grit sandpaper until it feels smooth to the touch. If the finish is especially rough you might have to sand it a little and add another coat and sand it again before you get the finish smooth. It's just very important not to sand through the finish. This is especially easy to do on the corners of the project. The sandpaper tends to try to round the corners which tends to take the finish off very quickly. One product that is very helpful hand sanding is a Glit sanding pad. https://jackpauhl.wordpress.com/tag/glit-sanding-pads/ These foam pads are easier to hold onto than sheets of sandpaper and also tend to hold the sanding surface more flat than sheets.


Okay awesome thank you for the info!

In a way I was looking for that glasslike finish but this was my first time doing a project so live and learn and it looks good! So if I wanted it to look glass like I'd use a grain filler, what exactly is a grain filler? Thank you in advance!


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## epicfail48 (Mar 27, 2014)

HarryS said:


> Okay awesome thank you for the info!
> 
> In a way I was looking for that glasslike finish but this was my first time doing a project so live and learn and it looks good! So if I wanted it to look glass like I'd use a grain filler, what exactly is a grain filler? Thank you in advance!


A grain filler is a material used to fill in the empty spaces in the pores of the wood. Picture something like oak, which has a very open grain structure. Its got all the little pockets lower than the surface, no matter how flat you get it, and you can't get a mirror finish on a surface like that. A grain filler it meant to sit in those pores and bring them level with the rest of the surface, so that once you have a finish on the finish isn't highlighting the voids. 

If you need a grain filler, I recommend timbermate. It may smell horrible, but it does work quite well. Its water based and stainable, so it matches your project. Word of warning, for the sake of your sense of smell, never use timbermate on red oak, least not without a respirator...


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## Steve Neul (Sep 2, 2011)

Think of grain filler like a very thin wood putty you can brush on like paint. This is how it's applied but after brushing it on you let it thicken into a paste and rub it into the grain with a coarse cloth like burlap. You could also use a squeegee like you clean the windshield of a car on flat surfaces. Most grain fillers only come in a natural color which you should have tinted to the color you need. Once the stuff dries it doesn't stain very well. I use this one which comes in several colors. http://www.mohawk-finishing.com/catalog_browse.asp?ictNbr=105 Still once you have the filler applied and you wipe off the excess a little stays on the surface and needs a light sanding before stain is used. 

Like I said earlier you can achieve a glass like finish with the polyurethane. You just have to fill the grain with the poly. On flat surfaces you can sand between coats with a hard rubber sanding block with the sandpaper of your choice and it will remove the finish on the surface and leave what is in the grain. Eventually in probably about four coats you should have the grain filled. If you do this allow ample drying time between coats. Any finish will dry enough to sand but will continue to dry and as it does will shrink. If you do this too fast you could get it perfect and then in a week or two the finish could shrink and make it look grainy again. With some finishes, shellac and lacquer even using a grain filler you can cause this. The solvents in the grain filler can make the filler swell up out of the grain and then you sand it flat and get it perfect and then with the finish completely dries the filler shrinks back into the grain and shows the texture again. When using lacquer I will only sand a finish with grain filler once a day. I allow any coats I put on dry overnight to allow the filler to shrink before doing any sanding.


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