Don't be concerned with the rust blue soln getting into screw and pin holes as others have said. It'll all work out just fine. The boiling water will turn the rust to bluing.
One thing you might have happen wether you are Slow Rust bluing or Express Rust,,when you pull the plate or any part from the boiling water and if the part has screw holes or other such pin alignment position holes in it,,these voids will trap the water in them.
The plate will quickly dry of it's own heat but the extra water trapped in the holes and voids may not evaporate right away.
That extra few drops of water can spill out a minute later when the plate is still very warm. The water drips and runs across the surface and now it drys up. But in doing so it often leaves behind a mineral streak outline where it 'dried up'.
The same mineral spotting can occur on the plate surface around the holes them selves as the water drys in them.
Yes there isn't supposed to be any minerals in the boiling water, but there often is or there are other contaminants or oil ,grease missed during the cleaning. It's just the way it is if you do enough rust bluing you'll know.
Blow the excess water out and off of the part. Skake the part gets rid of alot of it.
Some use compressed air to do this but make sure the compressor is 'oiless'.
I blow off the surfaces as best my asthmatic and sarcoidosis lugs will allow. Then dot any remaining water up with the tip of a twisted clean paper towel.
If you do get those contamination streaks on the surface, you can scrub that area with a piece of clean Scotchbrite to remove them. Even going down as far as clean steel again and feathering the polish out to the surrounding area.
The next application of Bluing Soln should be applied to the ENTIRE part. Do not try and touch up just the damaged and repolished area.
The color will come right back up to match the surrounding area in 1 or 2 cycles as the pores of the steel still have the bluing/rust in them.
Don't reuse that piece of Scotchbrite as it carries away the mineral or other contaminants.
I've never used any degreaser over the Scotchbrite'd area, Just polished it with it and went on with the bluing.
Works great on any rust bluing Cold or Express to save a job from streaks or discoloration and not having to quit and start all over again from scratch with strip and polishing.
