Dtm primer question.

The better the product, the less labor required in general. Look up what the pro body guys will say about filler. The high priced stuff is easier on them!! We all get caught up in cost of everything right, Me too! We will say why pay $80/ gal for filler when we can get stuff that is "pretty " good for $35. There is a difference.
Shrinkage, that is a big key, filler or build primers. Me I can not afford the best of the best, so I sand and respray and sand and respray, and after I figure it all out, I would have been just about as cheap with the best product in the long run.
Any place you can clean and prime and paint even with a heavy equipment enamel, LIKE inside the trunk extensions, Inside rockers if given the chance, inside the front fenders, inside the DOORS, is well worth the effort.
I personally get best results if I am not going to do the whole job at one time, to use dtm epoxy, and them come back over that with 2k urethane primer until I can finish the job.
There are many ways to skin a cat.