Morris Minor Owners Club

I have my radiator out at the moment and have done some repairs to the outer steel frame. It's in good condtion but I am wondering if it would be a good idea to spray paint into the core section to help stop rust?

yes it would but you need high temperature paint

A "light" coat with the HT paint rattle can...don't put on so many layers of paint that the cores get clogged up and fails to cool the engine effectively.
With the rad standing vertical, spray one coat slightly below the horizontal line and a second, above the notional horizontal line for the best coverage.A horizontal attack will blast paint through the fins hitting the metal fins end on.

Bob