*Start with a Sandblasted surface that is clean- or D.A. sand with 80 grit and clean. CLEAN CLEAN CLEAN!!
* Spray SPI epoxy, 2 coats with proper flash time in between. Wait 24 hours.
* Apply filler directly over un-sanded epoxy- fresh epoxy and filler have a death grip on each other
* After filler work is done, spray 2 more coats of epoxy to seal it in.
* before your 7 day window ( SPI only) is up, spray primer filler over project
By doing it this way you are ensuring proper moisture protection on either side of the filler. Epoxy and filler have a death grip on each other and making this " epoxy sandwich" is the best way to prevent rust from forming under the filler.
Any questions please ask away! I do this for a living.