I just finished sealing my asphalt driveway yesterday morning. Temps have been in the mid 90's and humid, and there's been no rain. As of this question, it's been about 36 hours since I sealed the driveway. I'm starting to see multiple stains like the picture below coming through the sealant. I think they were there before, but I thought they were just where the asphalt had worn more excessively. I'm not really concerned about looks. What is this and should I be concerned about it?
BMitch is right on his comment. This probably wasn't "dirt" but was some oil, rust, other chemicals... When you paint over them you dry them out and the result is a chemical reaction with the paint/stain because you are drying the material out - it cannot access air - and it will try to escape through the stains you see.
I know this because I paint my laundry floor every 3-4 years and I sweep it well but really don't feel like giving it a good cleaning. My fix is to just put more paint over it - which works sometimes on the first coat and sometimes 2-3 coats later. Is the paint bonded as good as it could be? No. But it is a floor. If you don't want to see these you have to do some serious cleaning before painting floors/driveways.
I have a 300 ft driveway, and I only have white spots like yours on the asphalt walkway from front door to driveway, about a 20 ft section. This is the only area i use rock salt on during the winter. The rest of the driveway is OK.
I found the easiest way to remove it is with a propane torch (plumbing type). Be careful not to overheat the spot as you will loosen up the asphalt. The amount of heat to apply depends on how thick the spot is; a thicker spot will turn yellow and can be brushed away, while thin spots will disappear.
It's a good idea to seal coat afterwards.
