The easy way. Use this marine grade cosmoline to coat the surface. It's tough and easy to apply.
https://www.amazon.com/CRC-06026-Heavy-Corrosion-Inhibitor/dp/B07N1VLTFD
Or
Obtain QUALITY stiff dull puty knife.
https://www.amazon.com/Hyde-Tools-01140-2-Inch-Stainless/dp/B001VEA860
Remove all LOOSE PAINT, debri, and rust, with moderate force scrape from the inside (rust epicenter) out (edges of paint) until you run into tightly adhered paint. Never paint over loose paint.
Clean surface with solvent until It's completely contaminat/dirt free, wipe surface with distilled water to get a normal ph on the metal. Dry the surface with lint free cloth.
Get a wire wheel with a right angle grinder, wear a respirator, safety GOOGLES, a face shield and hearing protection! Bust the rust till it's bare metal. Break out a 80 or 120 grit sanding disk, roughness the surface thoroughly to develop a surface profile.
Feather the edge with 240 grit sanding disk. If you're inexperienced, I suggest feathering by hand with 120. You want a smooth gradual transition from paint to primer to bare metal. The feathered area can be a few millimeters wide or around 1/4" roughly.
Photo #4 is a perfect example of a feathered edge.
1968 Mustang Fastback Resurrection: Underside Floor Finish and Door Striker Plates
Clean metal surface with a solvent until no more contaminants remain.
you'll end up using some sort of marine grade two part epoxy primer and paint if you want a durable coating. Use a respirator for fumes!
Lastly you'll need to find the required dry film thickness of both the primer and paint, a wet film thickness gauge will be required. Along with knowledge of percentage of solids by volume of the primer and paint. If a primer requires 4 to 6 mils dry, if the paint is 20% solvent. You need 5 to 7 mils wet. Also watch out for high humidity, if it's too high you can't paint.
Keep in mind, all epoxies have a pot life dependent on temperature, pot life is how long the paint is good after the two parts are mixed, it isn't long at all so DON'T mix the whole thing at once. Portion it out exactly as the manufacturer says.
SOME epoxies have an induction time, induction time is how long you must wait until the paint or primer is ready to use.