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Logical. I'm not certain three coats of primer are called for. I used two when I painted my cars, sanding between the two and lightly after the second just before the first color coat.
If you don't like what you see and feel after the second primer coat, you may need a third.
The number of primer coats really depends on what is needed to straighten out the panels. You may find once you start sanding, there may be areas where you break through and need to reprime.
You will get a feel for this and once you are happy with each panel, you can shoot a final prime coat for finish sanding.
Blocking out the car is the most critical for good results so don't use short cuts here.
I would suggest you let your painter apply the final sealer just before the base coat goes down.
To this day I cringe at the sight our sound of sand paper. I unsuspectingly walked in the room while my wife was filing a nail with an emery board and almost cried.
The primer I used was a 2k primer and you can mix a little reducer in it to use it as a sealer. Basically it thins it out so that it can get in to the cracks and pin holes better. After that I mixed and shot the regular primer.
When I block sand a car between primer / filler coats I start with 320 then primer again and again with 320 if need be. Then prime and sand with 400 if I am using metallic colors my last blocking is done with 800. If you are using a metallic paint the surface must be absolutely smooth or the metallic partials will flow into the little scratches that you can not see until you spray the car or panel you are painting. If you post this question on the “Paint and Body” section DUB will answer any and all your painting questions