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Using 220 grit, lightly, on a polyurethane finish (applied 2:1 poly:mineral spirits), left large and deep scratches all over the surface. I was concerned about it, but couldn't get the scratches out with 600 or 400 grit, and more 220 sanding was leaving more scratches. I tried applying another coat on top of that, hoping the scratches would disappear as the poly bonded with itself, but they showed through the top finish.

So, I sanded the finish down to get to the scratches, using 180, and then 220. I got to where the scratches were, but the 220 I used to sand down there was leaving more scratches.

I don't get it. I don't recall having this issue on prior coats when using 220. I've tried two different brands of 220, and they both are leaving significant scratches. I kept sanding on one area of a scratch, using 320 grit, to find out how deep it goes, and it seems to go all the way down to the wood. I'm thinking I'll have to completely remove the finish now.

I hadn't had this issue with the previous many coats I applied. Only after I made a new batch of 3:1 poly mineral spirits and let the first coat of that which I applied dry for a few days before sanding it. And, reading online, it sounds to me like sanding with 220 between poly coats is supposed to work fine.
 

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Using 220 grit, lightly, on a polyurethane finish (applied 2:1 poly:mineral spirits), left large and deep scratches all over the surface. I was concerned about it, but couldn't get the scratches out with 600 or 400 grit, and more 220 sanding was leaving more scratches. I tried applying another coat on top of that, hoping the scratches would disappear as the poly bonded with itself, but they showed through the top finish.

So, I sanded the finish down to get to the scratches, using 180, and then 220. I got to where the scratches were, but the 220 I used to sand down there was leaving more scratches.

I don't get it. I don't recall having this issue on prior coats when using 220. I've tried two different brands of 220, and they both are leaving significant scratches. I kept sanding on one area of a scratch, using 320 grit, to find out how deep it goes, and it seems to go all the way down to the wood. I'm thinking I'll have to completely remove the finish now.

I hadn't had this issue with the previous many coats I applied. Only after I made a new batch of 3:1 poly mineral spirits and let the first coat of that which I applied dry for a few days before sanding it. And, reading online, it sounds to me like sanding with 220 between poly coats is supposed to work fine.
I had a similar experience recently. After I sanded all the way back down to the wood, I found that the scratches were actually in the wood itself, they were just incredibly faint without a finish to highlight them. If you do go back down to wood, make sure you examine the wood carefully with different light sources or wipe it down with some mineral spirits to try to highlight any remaining scratches.
 

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The Nut in the Cellar
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My take is that 220 is too coarse for between applications, unless you are leaving noticeable brush marks. I use gray (000 steel wool equivalent) non-woven pads to scuff between applications, maybe green (00 steel wool) if the coat turned out badly. You only need to scuff the cured poly enough for the next application to bond with the cured application.
 

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Cabinetmaker
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Unfortunately, there is no easy way to fix scratches between dried polyurethane coats, without removing at least part or all of the existing finish. Also, you should only use very fine-grit sandpaper (320-grit sandpaper and finer) on polyurethane, when attempting to remove the scratched layer(s).

If you notice the scratch mark when re-applying the polyurethane over the sanded layer, you will need to re-sand the scratch marks from the affected layer(s) of poly. Do this when the poly has dried "thoroughly". Then, use 320-grit sandpaper (or finer) for the sanding. Remove the dust with a vacuum and a dry wipe with a cloth. Apply the next coat of polyurethane finish and let it dry, thoroughly. Once the poly is dried, if the scratches are still there, you will again need to sand down the finish.

I once experienced the same problem as you, on a project where a poly finish was used. I ended up sanding down to the bare wood, and then re-stained to blend the colors. My final move was to apply a "new can" of poly finish. With the new can, I never had the problem again. What was wrong with the original poly? Who knows.

You stated: "I hadn't had this issue with the previous many coats I applied. Only after I made a new batch of 3:1 poly mineral spirits and let the first coat of that which I applied dry for a few days before sanding it". -- Maybe there was a problem when mixing the new batch - or even with the remaining poly in the can?

In short, I'm a firm believer that it's not worth the time and effort to fix scratches between polyurethane coats, once the finish has dried. If you decide to start from scratch, I would try using a new can of polyurethane.

- Bob
 

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Using 220 grit, lightly, on a polyurethane finish (applied 2:1 poly:mineral spirits), left large and deep scratches all over the surface. I was concerned about it, but couldn't get the scratches out with 600 or 400 grit, and more 220 sanding was leaving more scratches. I tried applying another coat on top of that, hoping the scratches would disappear as the poly bonded with itself, but they showed through the top finish.

So, I sanded the finish down to get to the scratches, using 180, and then 220. I got to where the scratches were, but the 220 I used to sand down there was leaving more scratches.

I don't get it. I don't recall having this issue on prior coats when using 220. I've tried two different brands of 220, and they both are leaving significant scratches. I kept sanding on one area of a scratch, using 320 grit, to find out how deep it goes, and it seems to go all the way down to the wood. I'm thinking I'll have to completely remove the finish now.

I hadn't had this issue with the previous many coats I applied. Only after I made a new batch of 3:1 poly mineral spirits and let the first coat of that which I applied dry for a few days before sanding it. And, reading online, it sounds to me like sanding with 220 between poly coats is supposed to work fine.
How were you sanding? When I sand between coats I usually use 320 on a rubber block, or just folded on curves and corners. I do not really "sand", it is more like a wiping simply to knock off the nibs. I never power sand with a ROS or anything else between coats. Poly does not burn in like other finishes such as lacquer. Aggressive sanding can leave witness lines between coats.
 

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mike44
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My take is that 220 is too coarse for between applications, unless you are leaving noticeable brush marks. I use gray (000 steel wool equivalent) non-woven pads to scuff between applications, maybe green (00 steel wool) if the coat turned out badly. You only need to scuff the cured poly enough for the next application to bond with the cured application.
I am not an expert on finishing. I would like to point out that gray pads come in light gray and a darker gray. The darker gray is very coarse. Light gray is much finer.
Here is a list of various Scotchbrite pads.
7445- white pad 1000/1500 grit
7448- light gray 600/800 grit
6448 green pad 600/800 same as 7448
7447 maroon pad 320/400
6444 brown pad 280/320
7446 dark gray 150 grit
7440 tan pad 60 grit
Blue pad is about 1000 grit
I use several different pads , mostly on metal finishes.
mike
 

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I dont use pads, I only use sandpaper.
But anyway, I think the problems is if you start with 60 grit and then jump immediately to 150 grit, there is a heck of a lot more sanding required than you would suspect. That would take a large amount of sanding. Then you did it again with jumping from 150 to 280. I suspect a lot of the deep scratches from the 60 grit never got wiped out by the 150. and the 280 would haver a lot of work to do to wipe out the 150. Not nearly any where as much as the first jump. So, i think the 150 grit never really got the 60 grit sanded out. At least that would be my best guess with sandpaper.
 

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The Nut in the Cellar
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I get the non-woven pads from Woodcraft and they are Norton brand.
Green = 0 steel wool (I misspoke in my previous post)
Maroon = 00 steel wool
Gray = 000 steel wool
White = 0000 steel wool
I originally found the non-woven pads in an auto paint supply store but Woodcraft is easier to get to for me.
 
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Discussion Starter · #11 ·
What brand sanding paper? I believe I sand with 320 in between coats..
One of the sandpaper brands is Gator. I'm not sure what the other brand is.

Photos always help. Never used poly but many here do.
It'll be hard for me to get a good photo of it now. I've spent hours sanding it, and washed it a couple of times with mineral spirits. The washing with mineral spirits makes the scratches a lot fainter (but still visible in certain light and angles), but only until I lightly sand the area. As soon as some dust material is there, such as from sanding, they stand out clearly again.

For now, my arm is shot from sanding. I'll be working on it again in a day or two.
 

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Using 220 grit, lightly, on a polyurethane finish (applied 2:1 poly:mineral spirits), left large and deep scratches all over the surface. I was concerned about it, but couldn't get the scratches out with 600 or 400 grit, and more 220 sanding was leaving more scratches. I tried applying another coat on top of that, hoping the scratches would disappear as the poly bonded with itself, but they showed through the top finish.

So, I sanded the finish down to get to the scratches, using 180, and then 220. I got to where the scratches were, but the 220 I used to sand down there was leaving more scratches.

I don't get it. I don't recall having this issue on prior coats when using 220. I've tried two different brands of 220, and they both are leaving significant scratches. I kept sanding on one area of a scratch, using 320 grit, to find out how deep it goes, and it seems to go all the way down to the wood. I'm thinking I'll have to completely remove the finish now.

I hadn't had this issue with the previous many coats I applied. Only after I made a new batch of 3:1 poly mineral spirits and let the first coat of that which I applied dry for a few days before sanding it. And, reading online, it sounds to me like sanding with 220 between poly coats is supposed to work fine.
Two reasons come to mind. Perhaps the poly wasn't dry enough for between the coats sanding. Sometimes soft poly will ball up and make more or less a rock on the sandpaper and cause damage. Drying time is subjective. You can let a finish dry for a couple days but if it has been cool and or damp it might not be enough. What ever the directions say about drying time is for ideal conditions. The second issue is you may be thinning it too much. Any poly I purchase is thin enough to spray it right out of the can without thinning it. The more you thin a finish the more chances you will have of it showing scratches from between the coats sanding.
 

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Something pads are great for is getting rid of dust nibs..Slap one on a ROS and the nibs go away quick.. Remember to take the sandpaper off first.. :rolleyes:
 

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Hmm... things are getting cloudy. Reading through the responses makes me wonder how many threads we have going here. I presume you sanded the wood to at least 320 using a reasonable progression from where you started; e.g. 120, 150, 180, 220, 320 ... Scratches in the wood will not disappear under many topcoats, so you have to start out right, which I presume you did.
Then you applied poly thinned with 1/3 mineral spirits. That is a lot of thinner. Why so much? Was the poly really thick? If so, it probably should be replaced with a fresh can. Is that the right solvent? The wrong solvent can interfere with curing. Did you wait long enough before sanding? When you sand, the topcoat has to be cured, not just dry to the touch and all that thinner will slow curing time. Was the humidity high? Some oil finishes will not cure properly if applied when the humidity is too high, especially if sprayed.
I rarely use poly, but for any topcoat I sand with P320 between coats (both to level the finish and provide tooth for the next coat), and the final coat I take up to 1600 or 2000. Sure, 220 shows scratches, so does 320, 400, 600, even 800. If you don't want to see scratches, you need to go higher than 1000.
If you want no scratches, spray your final coat in a dust free environment under negative pressure and don't touch it with sandpaper. Lacking a cleanroom, I will continue to sand because my paint booth is my driveway.
 

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Something doesn’t sound right. You should have been able to sand out the 220 scratches. I agree with @SomeDudeOnline its likely in the wood.

Grey scotch pads are great for leveling out. I used that in the final coat to level out the finish prior to buffing. I’m not sure they leave a rough enough surface for poly. I don’t use poly much but I sand with 320.
 

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Discussion Starter · #16 ·
Hmm... things are getting cloudy. Reading through the responses makes me wonder how many threads we have going here. I presume you sanded the wood to at least 320 using a reasonable progression from where you started; e.g. 120, 150, 180, 220, 320 ... Scratches in the wood will not disappear under many topcoats, so you have to start out right, which I presume you did.
Yup. There were no scratches in the wood. The scratches appeared while sanding after the 4th or so coat.

Then you applied poly thinned with 1/3 mineral spirits. That is a lot of thinner. Why so much? Was the poly really thick? If so, it probably should be replaced with a fresh can.
The mixture I made is 2 portions of poly to 1 portion of mineral spirits.


I think I've just got to take it down to the wood and start over again. Next time, I won't use 220 between coats, but probably will stick to 400 and 600. I have 2000 to use for the 2nd-to-last coat. Unfortunately, the hot days have just ended here, and it's starting to be rainy and cool. I'm doing the work in my garage. Hopefully, it all works out!
 

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Discussion Starter · #18 ·
Why are you thinning it so much? I assume you’re spraying but that seems like a lot of thinning. What product?
I made a wipe-on mixture. Going off the sources I used, I didn't think 1/3rd poly was too much.

I've sanded the desk back down to the wood and gotten to the point where I've applied the first layer of poly. There are a whole lot more dust nibs in it this time than the previous time I applied the first coat. And I realised I forgot to vacuum the top before applying the poly. I wiped it down with a wet cloth, raising the grain, and then sanded those down. Then I wiped the counter as best I could, to remove dust, but I didn't vacuum it. Will that be problem, and should I sand it back down to wood, and vacuum it before starting with the poly, or should I just sand down the nibs and carry on?
 

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Discussion Starter · #20 ·
I've sanded out all the dust nibs in the first coat of poly. I used 400 grit, and it took some time. But it looks and feels good.

I'm hoping to get the finish right, this time. With my last attempt, before sanding back down to wood, when I was on maybe coat #4, it was getting an orange-peel effect, and it appeared that the finish was being sucked into all the darker fine strands of grain. I sanded the desk down with 180 and 220 after that, and then smoothed with 400 and 600, and then re-applied a coat of finish. That coat looked great, the best yet - apart from the visible scratches.
 
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