Almost all rust (esp on cills / tailgate and other double skin areas) comes from the INSIDE. Treating the outer surface will make it look ok for a while - but it will be back.
That said there is bugger all else you can really do other that be a bit vigorous digging into the rust holes ---- if they enlarge well beyond expectations (especially on the cill) life can be tedious/ tricky and the writing is on the wall so to speak.
If you DO end up with a fair sized hole in the cill (say 5p size) then screw up a piece of aluminium gauze with a bit of strong thread tied to it, push the gauze through the hole and pull the thresd so the gauze is tight against the back of the hole. Now you can apply filler over the gauze. Rub down, paint etc. If you are lucky that will not be spotted on a MOT. Anything more than 5p size = go for a replacement cill.
Been there. Done this. We all have probably
5 Precepts of Buddhism seem appropriate. Refrain from taking life. Refrain from taking that which is not given. Refrain from misconduct. Refrain from lying. Refrain from intoxicants which lead to loss of mindfulness