I've looked at archived threads on cold rust blue processes (Art's Belgium Blue), and people who have used it seem to be satisfied with the results. How do the results compare with "traditional" rust blue in terms of color and durability? Thanks

Not sure of just which blues you are inquiring about. the old traditional "Rust Blue" is a Cold Rust process. The Belgian Blue I am familar with is an "Express" (Hot Rust) blue. The Touch-up blues are often referred to just as "Cold Blue" but do not normally include the term Rust. Then there is the Nitre Blue & Hot Salts Blue. Of all these my personal favorite is the Cold Rust. The Nitre Blue was often used in conjunction with a Cold Rust one for such parts a trigger guards, screw heads etc with the barrel having the rust process.

Sorry if I was unclear. I mean the "Express" blue process.

I have always preferred a rust blue finish. The finish seems to have more depth to it than a hot blue process. My understanding is a rust blue will hold up better than a hot blue, but others with more knowledge will hopefully come along to answer your questions. How well either is done will also make a difference. Doug Turnbull charges $250 to rust blue a double and you can probably get a good job done for less.

I think the Mark Lee stuff is called an "express" bluing solution and, altho awfully fast, it must be an oxidizer as it produces a brown or orange deposit which turns black in boiling water. What could that deposit be if it isn't rust? For a few minutes work it stays where you put it better than the phosphate depositors like Oxpho Blu.