Why is my Rust Mort not working

[ckissick]

I've been using Rust Mort on surface rust and it's always been great. I'm doing the floor pans now. I sand and wire-wheel as much rust away as I can, then apply the Rust Mort and it all turns black. Until yesterday. Only about half the rust turned black. The other half remained unchanged, mostly in the pits. The part of the pan I did the day before turned out fine.

The only change has been the weather. It was cool and rainy yesterday and cold in the garage. Does this process require low humidity and warm temps? Will repeated applications eventually get it all?

[ckissick]

Here's a pic about 2 hours after a second application.

[eastbay]

Check your label, most of those require 70 and higher. I have had better results with plain old phosphoric acid, cheaper too.