Like the other guy said, most profesional body shops will only do
entire panels. Some insist on doing the ones next to a damaged panel so
that they can 'blend in' the new and old paintwork (they do this by
fading out the spray and then compounding the panel back to a high
gloss). But it sounds like you are working with an aerosol spray system
and so this approach is not realistic.
As a method of avoiding a lip between the newly painted area and the old
paint, you could lift the edge of the masking tape by 4mm or so. This
help to prevent a sharp edge. However, the paint underneath the lifted
masking tape will be slightly dustly looking rather than super shiny.
You can get rid of this (after a week or so) by first sanding the whole
area with 1200 grit wet and dry paper and then polishing with a cutting
compound such as Farecla G3 using a Gmop applicator fitted to a drill.
For a supplier see:
Wicks do a M14 adaptor so that you can fit the Gmop into a drill.
If your old paintwork is a bit faded you may well have to compound the
entire roof to blend the new area in with the old but you should be left
with a no visible lines between the new and old paintwork.
Hope this helps
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