The issue become quite real when one considers what takes place when the painter "blends" into the existing clear with todays low VOC materials and the importance of having sufficent UV blockers in the top .5 mil of the clear.
Blending does not allow enough film build, IE thickness with enough UV blockers at the edge of the blend.
As well, it usually requires some wetsanding and buffing of the repair area so as to match the original finish that the blend is applied to.
Over time and enough UV sunlight exporsure, a "cloudiness" will start to show at the blend edge.
This is due to the factors I mentioned, not enough film build, not enough UV blockers in the material at that area.
Over time, this defect may well start to expand into the spot repair.
Old, high VOC materials, different chemistry of the materials, blends could often last for years without exhibiting any noticable change in the blend line.
Of course the cavet on this is the grade of materials, surface prep, amount of sunlight hitting the repaired area, etc, etc.
Does a blend always fail? Of course not, however, in the larger percentage, failure will happen.
This is why that PPG, DuPont, etc will not warranty their materials for such failure when a blend is done.
Complete panel refinish to a panel break is required.
Grumpy