[ByByeAmericanPie]

Another way to prevent the picture frame effect in dealing with a small area is to not mask at all if it is in a location that allows you to do so. Apply the clear to your area of interest and then gradually feather it out to the edges.

Then when you come back and do your final polish out (and you must do this final polish with a medium cut glaze like Meguiars #80 Speed Glaze applied by machine), you can blend back into the existing clear coat easily without having to deal with straight edges of unequal thickness.