This is a fairly old thread but here's my 2p worth.
Rust is frankly a B*****d to fully remove, as some have said a professional is probably the way to to go but there are some things you can do.
1) As some have told you before that sanding away the area is bad because it will remove the Zinc Galvanisation, I disagree. If the area is already corroding it will continue to do so.
2) Rust converters do have their place, however they can only do so much. If your prep is right they can help but slapping it on won't do much. A more methodical approach is needed.
3) While the rust on your car is by no standard 'terminal', if not treated right it's going to become a bigger and bigger problem.
In your shoes, I'll use the 3rd picture as a example, what I would do is:
*Ideally cutting the metal out and welding new metal in is preferable but this is fairly specialist and a faff to do. Can be expensive if a garage does it. If not then:
What you need: Hilt Hamber Rust Treatment, IPA (Rubbing Alcohol, not the beer [emoji6]), Acid Etch Primer, Angle grinder and disk/Dremel multitool, undercoat wax, Paint of your choice of colour, oil can or sprayer, seam seal or equivalent....see below, rubber bungs, paint brushes.
*Grind back using a grinding disk on a angle grinder (a dremel with a flexible drive tip is a beauty for this sort of job to help access too), get as much rust and paint off the area as you can, leaving the area shiny. Grind off any paint around 1 inch past the rust and onto the healthy steel.
*remove the seam sealer either by grinding out or cutting.
*The next step depends on if you can access the back of the panel or not. If you can, repeat the process so far and the rest of the steps on the behind area. If not, heat the area with a hairdryer for a good 10 mins at the minimum, rust and moisture is a match made in hell. This should heat the back of the panel well. Using a oil can/sprayer/ angled brush, use the space where the seam seal was (or drill a hole to get access around the area, get rubber bungs to fill the hole afterwards) and cover the back with a waxoil/undercoating wax. While this won't kill any rust behind, if it's been dried well enough, you will not be trapping any moisture in the rust and therefore if sufficiently covered will slow it dramatically or stall it.
*Use some IPA on a cloth to remove the grinding residue or paint dust left behind. Next use Bilt Hamber rust remedy on the area of bare metal. While as we said it's not a miracle cure, if sufficient bare metal is showing and only slight rust after grinding, it will work well. Follow the instructions on the bottle. I've previously found doing a coat, leaving to dry, grinding it back and doing 2 further coats is far superior to any other method.
*Once this is done, get a beer and a smoke if that is your poison, have a breather.
*Wipe down with IPA again, this will give a clean surface and help adhesion. Now, get some Acid Etch primer. Give the area 2 to 3 very light coats, doing the same for any holes you have cut or made, leaving a good hour in between. Check the area and make sure your happy with the coverage.
*Once again, IPA wipedown, then Paint the area with your choice of colour. Personally on most underbody/underbonnet rust repairs I would suggest using black stonechip, as it's colour helps when seeing if rust is trying to make a comeback.
*Now leave the area for 24 hours. Once this has been done, check the area and see if your happy. If you are, check the back of area, if you have wax coated the back of the panel, now is a good time to do so again, 2 coats are better than one in that situation. Once all is done, fill any holes you have made with rubber bungs. Reapply seam seal to the area which initially had it. While it's normally under the paint on a car, as you are fixing the paint and preventing its contact with air or water, it's best to do it on top of the paint. It won't do any harm. Most automotive trade suppliers sell seam seal, if not a good trick of the trade is to use flexible bathroom sealant. While not ideal it does the job very well indeed!
Now you are good to go.
I've used this method on a few cars and unless the rust is really bad this should treat it/slow it down massively. If the rust has left gaping holes in the steel then the only choice is to to cut it out, otherwise this could be a solution.
Hope that helps, if not you maybe someone else [emoji106]