It is sad but true that we all deteriorate and weaken with age and the same goes for paint systems. The first coat of paint that is applied to a substrate will inevitably become the oldest as new repaints are applied on top of it. Eventually that old first coat will just become too tired and simply let go and, when it goes, everything above it goes as well.
Picking just exactly when built-up paint systems require an expensive total removal is a black art because failure of the underlying system can be catastrophic with few early clues. An old original primer might be coping under the burden of several repaints but the additional stess of applying yet another repaint could be the straw that breaks the camel's back.
The old primer lets go of the substrate in a series of blisters that go right back to the original surface. The fault is not the latest topcoat, but that the old primer can no longer perform.
The presence of these blisters is a sure sign that total removal is necessary as patching up is only a palliative. There may, however, only be a constrained area that needs this treatment - areas that have had the most exposure to the weather.
The need for total removal can sometimes be predicted by checking the adhesion of the system with special adhesive tapes. In other cases the problem may only become known after the topcoat has been applied and left to weather.
You can use the following tape test to indicate how well the paint is adhered:
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