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You have better skip that particular rust converter -- they are not all created equal. There are also none specifically recommended for cast because of the lifting problem you described. HOWEVER, there is a product called Picklex 20, and another much cheaper product called Must for Rust that I bought at Home Depot -- so far they appear to me to do the same thing. You apply with a squirt bottle, even it out with a brush, wipe off all the excess and let it react at least overnight. When I use it it is more like several days. You do not wash it off or, as you noticed, you get instant rust. When the stuff is dry, I remove all the residue I can with a scotch brite pad, then clean the metal as good as I can with soap and water, 409, or other household cleaner -- dry it before rust forms! Then I treat with multiple applications of wax and grease remover until the rags are clean. I then assume that most of it has been removed, then I use epoxy primer followed by topcoat. I have done 3 tractors this way with no apparant problem, but I do scrub it good until I get what I consider to be more or less complete removal. You will never completely get it out of the pores, that is why your paint lifted. Use it next time you paint, but I would not use it at present, you may end up getting too much acid. I would remove the primer and scrub the pee out of it, forget reapplication and prime again. It sounds like the product you are using is suitable for new sheet metal, not cast, and is a substitute for sanding. If you wirebrush it good, then scrub and dry, then use wax and grease remover and prime, I doubt if you will have any rust problems, provided you prime within a reasonable amount of time -- that depends on your climate and humidity, and whether you are working inside or outside. If outside, prime immediately after cleaning. If you try to spot treat it, the primer/topcoat may lift in days/weeks/months, I simply don't know.
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