I used evaporust on my GS tank. it did soften and discolor the paint some. I used it on an old rusted license plate and it worked great.
Ever try Evaporust on painted surfaces? I've read it can work well on both painted and raw surfaces, but have no experience.
in both cases it was not bad and ospho may have been better or worse. Oshpo does protect the clean metal surface better, so after the evaporust I'd still hit it with ospho.