Gary has nailed it!
When you deal with this type of work everyday, you take it for granted or think that everyone knows this already.
But, it is all about "Time" when you are paying someone else.
Your build may be "Your" hobby, but the Sandblasting and Painting is a "Business" to the person you are paying!
Personally I would strip, repair and prep the chassis as best I could (no grease) before I took it for blasting. Then I would prime and paint it myself (but I am set up to paint).
Painting can be fun and gratifying for you, as well as a way to save money because you are not paying for "Time" unless there is something else you can be doing with your time that makes you more money than the guy you are paying to do your work for you??
It is just about the math.
Good Roads
Brian & Sandi