You can top-coat/overcoat/primer POR within a certain short period of time.
If outside that period of time you need to apply a "tie" coat.
Will your frame "see" much UV-rays/sunlight?
From POR website:- Topcoat when first coat is dry but with slight finger-drag remaining, usually 4-5 hours.
- TO TOPCOAT A FULLY CURED POR-15 SURFACE:
Wet sand with 320 grit until gloss is dull, then paint, or use POR-15 Tie-Coat Primer directly on the cured POR-15 surface, then topcoat as desired.