I would take a wire wheel attachment on an angle grinder... Safety Goggles and long sleeve clothing and wire wheel the rust off.
Wipe down with Acetone rag several times. Paint on POR15 to the parts that had rust starting...
Let dry.
And then paint them...
For paint VHT Satin Black Epoxy Paint. Then I will buy some bed liner aerosol and paint only the steps if you have steps...
Amazon.com: VHT SP652 Satin Black Epoxy All Weather Paint Can - 11 oz.: Automotive
Once I confirm my order for a 2015 4Runner, I'll be making an order for OPOR sliders with the diamond plate steps for the front and rear doors... I will clean them up with wire wheel (in case there is welding slag) and then paint using the VHT paint I mentioned.
I thought about doing 2 steps... epoxy primer (not cheap), then painting wrinkle paint over it.
But the VHT epoxy satin paint looks like it will do the trick and is cheap + readily available.