eric: Mr surfacer out of the jar makes what is in my opinion, a terrible mess. i tried thinning and airbrushing Mr surfacer 1000 the other day, and it went on great. but golly cleanup was a total !@#$%. the stuff doesnt dissolve and flow away like paint. it makes flakes and the flakes go all over the place.
to answer your question : Mr surfacer can be cleaned up using acrylic thinner or anything hotter than that, but i reccomend getting it in a spray can for convienience, its not too much more expensive, factoring in the cost of thinning and wasteage and water and more thinner needed for cleanup.
your best bet for seam-filling is a toothpick. for larger surfaces how about a Qtip? no cleanup, just chuck it in the bin. apart from the cleanup mess, Mr surfacer is the best thing since sliced bread foor seam cleanup, you can even use it to fill minor sinkmarks, and basically even treat it like a putty ! its great.