Don't be tempted to leave it in Evaporust for more than two or three hours at a time, you may get a reaction known as "Carbon Surfacing".
Have a look at this video by Joe Pie. Start at 0:50 onwards as he talks about Evaporust and some of the "issues" associated with long submersion of items in the solution.
I already did a test of spring steel in evapo rust, I didn't have a problem.I've left a cast iron part in for over a year before, leading to extreme pitting on the surface.
I usually leave parts soaking for 24 hours... it comes out clean with no damage at all.
Never use evaporust on any type of spring, though, unless you want it broken...
-Bear