I would finish all of the bodywork and sanding, then choose a non-sanding primer-sealer compatible with the topcoat to seal the Rustoleum, then a sanding primer if you need to fine tune the surface prior to topcoat.

1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy