Acetone is typically used when there's just a few small spots or small areas with moisture trapped in the surface of the sealer. A little scrubbing with a rag or brush will open up the surface enough and allow the moisture to escape and then dry back down clear.
Xylene is typically used when there's a major amount of moisture trapped and/or the moisture is deep, for instance, if the 2nd coat of sealer was applied before the first coat had completely dried clear, or if applied too heavily, or if applied during very high humidity thereby trapping the moisture deep into the sealer. If the moisture is over a major part of the area but it's trapped in the surface and not deep, simply spraying the xylene onto the surface will do it because the xylene will quickly open the sealer up and encourage the moisture to escape and then dry back down clear. This works many times for moisture trapped deep as well.
If just spraying on Xylene doesn't work, then you'll need to use scrub brushes with the xylene and strip the sealer off, so you can begin again.
Remember that if the temperature is cold or the humidity is high then that slows and/or prohibits the moisture from escaping. However, if it's very hot then the sealer wants to dry back down quicker, perhaps faster than the moisture can escape. So what you want is a warm temp and low humidity as best as you can get it. The humidity is the primary concern. 40% to 50% is a good range, lower is better, but 60% is pushing it and 65% to 70% is a deal breaker. For ideal temps, think tropical. 70 to 80 degrees is great, 90 to 95 or so on the high side is about it, and on the low side, 60 degrees is ok but 50 degrees is pushing it and 45 is absolutely the lowest.
If you just need to spot scrub only for smaller areas, then after it's dry you may want to apply a bit more sealer and feather it out to smooth it out. You'll know once it's dry.
For areas that are just sprayed, you likely won't need to apply any more sealer. Unless you have dry or patchy looking areas once it's dried. Then you'd need to do another overall coat.
For areas that are scrubbed and the sealer is stripped off, you'd need to do 2 coats as you'd be resealing from scratch.
Acetone and Xylene (Xylene, Xylol, Touline) are flammable and aromatic hazardous products but they are overwhelmingly what we've heard works best through the years. You can of course try using safer alternatives such as citrus strippers etc., regardless of what you use though, be sure and read all instructions, warnings, cautions and complete labels. Familiarize
yourself with the products you're using and ensure plenty of ventilation and common sense and keep your work environment safe and productive.
I hope this helps,