Rust on Frame...How do I mitigate further damage?

Recently purchased an '02 Tacoma and noticed a rust spot on the frame. What's the best way to tackle this to prevent it from migrating? Added some photos for reference.

 

Scrape and wire wheel the rust off. The big flakes and all. Then hit it with a good primer. Followed by a good flat black paint. At least two coats. Then apply something like FluidFilm.

Fair warning. It will look BAD when you start scraping and brushing the rust. But ALL the rust had to be gone. Otherwise, you’re wasting your time.

 

Any recommendations on primer / paint? Is there automotive-grade stuff I should look for?

 

Eastwood Extreme Chassis is good stuff.

But I've had good results on my trailers with Rust-Oleum too.
The key is, don't spray over ANY rust with it. It won't adhere to the rust at all.

The Eastwood stuff has an epoxy agent in it. It "sticks" better. But the same principles apply, not to apply it over rust.

 

Rust-O-Leum's rusty metal primer is pretty good. But I don't think it's as good as it was when they actually added fish oil to the base, but that's just my opinion.

I used the stuff to save the flaked and rusted out floor of an old Jeep Cherokee. I had got to the point where wire brushing was just polishing the rust. I managed to remove all the flakes. First, I brushed on a coat diluted 50-50 with paint thinner, and let it set for close to a week. That was to let the thinned paint soak into the porous rust, as best that it could. After the hideous smell dissipated and the thinned coat dried, I brushed on two more coats at full strength. I eventually gutted the interior, except for the dashboard, and had the Cherokee painted inside and out. The repair work held up well for the decade that I owned it.

It's important to deal with all of the rust. There is likely to be more rust on the inside or back side of the frame that will also need attention. It's difficult in the boxed sections of the frame, though.