MP170 is certainly capable of being applied very smoothly, providing the underlying surface is smooth to begin with. It should not require additional sanding or scuffing prior to topcoat, if within the time window on a proper surface. If you are unable to apply the 170 as smooth as you would like, look to your application process for the reason. The 170 tech sheet permits reduction of up to 10% with acetone if needed to facilitate a smooth application.
Depending on the resulting profile from your sandblasting, more than one coat may be required. Or, as Stephen recommends, a sandable surfacer may be used. Two coats of the 170 should provide a decent surface for direct topcoating over a reasonable profile. At least it has for me.
Check the wording in the latest edition of the tech sheet and make sure you are not approaching the pot life before use. You should be able to get the latest tech sheet (pdf file) here.