Tape line

You could try using superfine grit polishing cloths or papers to smooth that out with very light pressure.

When masking like that, I try to spray away from the tape, not into it, to help avoid that issue but sometimes it is inevitable.

When you spray over your mask do it in 2 or 3 light coats and spray straight down or from behind your mask never spray aganist the edge that you want left. Then when you pull your mask pull the tape back over itself at a low sharp angle. If there is a ridge there it will come off with the tape.    ACESES5

Thanks for the useful tips

Depends on whether paint is gloss or flat.  With gloss paint superfine sand paper or polishing cloth alone will do job.  However, that screws up flat finish, so flats need a dull coat after polishing. In fact, if the ridge isn't TOO thick, sometimes clear coats themselves reduce the visibililty of the line.

Another approach for future use is the use of Silly Putty which you can get at Walmart. This stuff works great for masking. You can create hard demarcation lines or soft ones depending on how you lay the putty down. When done just pull it up at a sharp angle away from the "fresh painted surface" . Just go with Dre and ACES advice on the direction of the paint sprayed from the airbrush.