When I bought my home last summer, I noted that the front of the house and the garage were a slightly different color than the rest of the exterior. My guess is that prior to selling the house to the owners before me, which would be 4 years ago, the then owner tried to improve its appearance by slathering paint on the front and garage, leaving the less noticed areas undone.
I've come across an old can of paint in the garage that matches the front/garage parts of the house. Its (non-oil based) primer. This would make sense, because the paint on these parts of the house doesn't look like top coat.
Assuming that these areas are old primer, how should I handle these areas? Should I reprime the whole surface? Or can I just TSP clean, spot prime, etc.