How should I address my lower quarter rust on a budget?

mine went there, rotted from inside out due to poor rear window seal.............i fannyed around with it then one day got my grinder with cutting disk, placed it against the body, closed my eyes,wimpered a bit then cut cut cut!!! im doing same now with a rear wheel arch,only way really! good luck whatever you do, my thinking ive filled mine here and there over years, it always comes back.i want a half decent paint job but pointless if the rot comes thro after 6 months.