Yes, you can apply NuMetal Epoxy Putty over cured RustSeal (24 hours) by scuffing up the RustSeal with 320 grit (or similar) and then applying the NuMetal Epoxy Putty.
TIP: Sealing up a metal surface first with RustSeal before applying NuMetal is a great idea, so rust does not form below the NuMetal and potentially cause it to pop off.
Please read the complete NuMetal Application Directions before you begin your project.