Some rust converter products leave a sticky residue & directions will have you rinse the product off, this will promote flash rusting; Do not do it. If working on an exterior panel use the product as directed then dilute the mixture to mostly water and wipe the residue off, before it drys. On your roof the residue will not matter, but make sure the product does not need to be removed or neutralized. For me, I like “OSPHO“. You can get it at Home Depot; you put it on full strength and leave it on to dry. Also, it can be painted over or top coated with “Boom mat” sound deadening spray or similar stuff.
SEM also makes a ready to spray etching primer, it’s excellent. Etching primer is good for light surface rust on body panels & parts, but in this case with the heavier rust I would still recommend OSPHO.