You don't know the half of it TS.
Besides shrinking, different fillers have different effects on the primer and paint. You need to allow all of the residual outgassing time. Even primer shrinks into scratches and pinholes. I use auto "glazing putty" for the pinholes in the primer, then prime again.
I seal MDF first with sanding sealer. Don't know, if I had better primer it may not be necessary.
Spray cans just don't have enough solids in them. I would rather not go to full automotive primers, but they will do the job. I need to look for a full body, plain old enamel primer.
Those million dollar car paint jobs? Part of it is because they let it cure for months between prep and paint. ( tells you something about the reality TV shows doing a car in a week)