I use "drip shavers" or "run files". Just did this on an MG I sprayed when it was too cold. Had a few sag/runs to take out. I let the paint (catalyized) cure for a week or two to let the paint harden. The drip shavers work very well to skin off just the run. Leaves you with a flat enough transition between run and paint that a little wet sanding with 1200 will smooth everything out.
I'll get some pics if you want. I got the drip shavers at Harbor Freight.