So... I assume you've painted and clear coated the entire piece and now you want to add another color.
If you tape off an area on the clear, scuff inside that area then paint and clear it you will surely get a tape "hump"!
Situations like this called for taping the base coat after a couple of hours, spraying the second color, remove tape, take your finger and rub the edge to remove loose build up and then clear over all.
To reduce the tape "edge" really will require wet sanding the whole piece and then doing the above. If not, you will have the edge hump!
Oh dear it's going to be a lot of work soon!!!