featherd edge with a spray can?

Just pull the edge of the tape up so its not stuck along the line.
Then spray so you arn't spraying directly under the mask.
it'll leave a nice feather effect.

You can also cut your camo patterns from light cardboard / heavy paper and use small rolls of masking tape, Blue Tack, something like that to hold it up off the surface of the model. The space will allow some of the overspray to get underneath producing a feathered edge. The higher the patterns are off the surface the heavier the feathering effect.

Practice on some scrap though. It works well, but takes a little adjusting to get everything the way you want it to be.

i use the old "rolled" blue tak/elmer's tak/poster putty method. roll it into about a 1/4 inch "rope", and then press it against the subject in the pattern you want. the trick here is to keep the spray pattern at the same angle with each pass to insure a nice, evenly feathered edge. good luck. later.