I'm working on a 39 ford tudor that has some pretty decent surface rust on the roof and hood. I first sanded everything down with 40 grit on a DA sander. After that I wire wheeled it real good to clean out the pits. After that, I applied some Jasco Prep and Prime which is phosporic acid. I can see that the acid has done 3 things: dissolved some rust, turned some rust black, and left some chalky white deposits. Of course, I'd like for all the rust to be gone, but I'm okay with it turning black, which is iron phosphate from what I've read and it's okay to paint over. However, I can't get the white scaly stuff off, I've tried more 40 grit on the DA, more wire wheeling, stripping discs on angle grinder, nothing is removing it. Is it okay to paint over? If not, how can I get it off? I have a sandblaster as a last resort, but I'm really apprehensive about using it on a big open panel like the roof.