Just speaking for myself , i use a metal etching primer on the undercarriage first ( SUCH AS VERIPRIME ), Then i will fix the minor imperfections using a light THIN coat of ALL METAL - Its a aluminum base body filler..... Then coat as needed with the DP90 EPOXY .

Im just getting ready to do it on this car.
HERES A PICTURE OF THE 69 CAMARO I AM WORKING ON AT THIS TIME.