We prime almost all of our routed and sculpted work for a number of reasons. The first is to introduce subtle texture. We use FSC-88 WB. The WB stands for water based. It is a Coastal Enterprises product. FSC-88 WB is a sandable, thick bodied primer. We generally brush it on our projects with a small brush, most often a 1" fitch and purposely leave our random brush strokes behind as we work. We do not sand them out later. The brush strokes along with all the other texture we purposely create in the manufacture of our projects makes everything look hand made - even if we used our CNC router to do the bulk of the work.
The primer also evens out the surface texture of different substrates. We often use sculpting epoxy to create elements of our projects and mount them to other parts made from 30 lb Precision Board. The heavy weight HDU is not nearly as porous as the lighter weights most shops use but it is different in texture than the sculpted epoxy. The heavy bodied primer takes care of that in only one coat.
The brilliant white primer also makes the subsequent applications of paint easier to apply and the colors appear more intense with less coats of paint. Making our projects unique is a series of little steps that add up.