I would say, a coat of primer would be good, anyway. Otherwise if you take long to finalize all the fixings, you'll get surface rust that will take two days to clean.
I painted my chassis after I've made mounts for the engine, gearbox, and radiator. I've had to strip and repaint in 3 places: bracket for the T-union for the brake pressure switch, relocate brackets for the inboard rear brake hose mounts, plus brackets on the rear uprights for the short S-shaped brake tube pieces. Grinding and repainting was not a big deal to me.
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