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Also before the finish is applied, the floor is completely vacuumed and then wiped with a thinner moist tack cloth...this shows if any trouble areas were missed in the sanding operation. and
Since you have three coats down...you next best option would be to "screen" the floor and then use a tack cloth after vacuuming. The screening will dull the finish a bit so you will need to apply a 4 th coat. I would screen with a 180 or 220 grit.
I normally use a 4" wide china bristle brush, and work from my hands and knees...with a gloss finish is it important to use good low angle light to ensure you do not miss some spots. A lot of folks use a lambs wool pad...but you need to be careful not to over apply
When I screen I use a standard 17" floor orbital with a red buff pad between the brush driver and the sanding screen...this is more gentle than a hard rubber faced sanding disc.
Here is an article I wrote with some extra Info
Using finer and finer sand paper as you go before the finish is important.
Once it is sanded, then tack cloth clean and apply the first stain coat. Once dry, a light sanding again then clean and tack cloth.
Next coat, same pattern, light sand, clean,
Final coat then should go on using a lambs wool applicator, that has been washed if new prior to using. Lots of lint will come off and you do not want that in your finish.
Sense you now have a finish on it, You should do as KMS talked about, using a screen sander with a fine mesh, you go over the entire floor with it. This will remove just the top coat of finish if you do it carefully. Once done, clean real well, use a tack cloth and then apply your final finish coat on the surface.
The only other thing I would tell you is that, if the finish is fairly fresh, it will tend to gum up the sanding screen, so be sure that you have enough on hand and change them out as soon as the gum up process begins. Otherwise you will end up with scratches all over the floor.
We rented a drum sander and started with a 36 grit followed by an 80 grit and finished with a 120 grit.The wood felt as smooth as glass before the first coat of polyurethane was put down.We are using Minwax oil base gloss polyurethane for floors.The can says no sanding between coats required.we are going to put a fourth coat on this evening but I will lightly sand and be sure it`s very clean
Also using a roller will cause bubbles to form on the sealer as well. That may have been the issue all along. Good luck and let us know how you make out.