I have found that great care has to be taken when sanding a run. You must sand ONLY the run, it is very important to sand ONLY the run. Think about it, you have three coats of clear right next to that run, the run it's self is the thickness of 6-10 coats of clear. So while sanding that 6-10 coats you can't sand the surrounding area that is only 3 coats at all. If you only sand it a little bit, a little bit of the 6-10 coats is your 3 coats and you are THRU like right now.
So using a hard narrow block where you are ONLY sanding the run and keep an eye on it by squeegeeing off the water so you can see what you are doing often you can keep from sanding the surrounding area.
These 3M squeegees work the best I have found. They make all different sizes so you can get into tight areas.
3M Wetordry Rubber Squeegee, 05518, 2 in x 3 in-Shop 3M
Keeping an eye on what you are sanding is the best advice I can give. Sanding that run without paying very close attention and you will wipe off the water and residue to see that you have sanded thru the clear sure as poop stinks.