Removing runs is something that every painter has to deal with. No matter how many years you put in no matter how many cars you paint the fact is you are going to have an off day and cause some runs.
Really this is not the worst thing that can happen but it does delay you and if the run is in the color coat it will mean stopping the work until you can break out a uv light and cure the work for sanding or maybe wipe down the area with reducer and feather it in.
If your run is in the clear coat then you can simply finish your work and fix it later by sanding and buffing.
There are many tools to remove the excess paint of a run. I like to use a brand new single edge razor blade to cut down some of the bulk but there is a relatively new tool or one that I have seen used more lately that can maybe to better then a single razor.
They are called nib tools or run razors. Basically what they are is a about 2 inch square cut piece of an aggressive wood type file that is mounted in a wood block.
They are used to remove dirt and runs and if used correctly on the bulk of the problem can cut back a lot of the problem you need to remove prior to finishing with sanding.
My suggestion is if you are going to use one of these tools you will probably be better off if you stick to those really heavy runs and you most likely don’t want to try to file the wing of a gnat out of the center of your hood.
The cost of these things vary and I have seen single ones run around $35 and maybe more.
Normally you can get a set of fine, medium, coarse and this is not a bad idea because the manufacturer is giving you some variances in the aggressiveness of the files. You will probably end up favoring one and not using the others but if the set is cheap you might as well get them all.
To use the nib file you place it flat on the surface and make sure it is only contacting the run.
You want to make sure that when you work with it you aren’t digging a corner into the good paint so use it flat.
Remember that you don’t want to use them to completely cure the problem with a light touch with 3000 grit .. they are only to take off the bulk of the run and then you need to finish with a rubber block and some paper.