As found, the motor cheeks om my Lionel 1688 were a bit rusty with bare patches of original metal blackening. I didn't want to paint them ... too thick and prone to chipping. I tried a "re-color" with a black Sharpie pen, but it was too blotchy with an odd bluish streaky sheen. (I removed that failed attempt with some denatured alcohol.)
I've seen some other guys here on the forum use gun blue, so I thought I'd give it a shot. I bought a ittle 3 oz. bottle of Birchwood Casey Super Blue, and tried it out. I first cleaned / buffed the metal cheeks with a stainless Dremel brush, then applied the gun blue fluid with a little cotton rag.
To my surprise, the gun blue fluid is mostly clear in color. However, the metal turns black almost instantaneously when you rub the fluid on it. I applied 2 or 3 light "coats", just a minute or two apart. Then, I let that dry (just a few more minutes), then "polished" it (sort of) with a fine ScotchBrite pad. The result is a nice near-original-looking black sheen.
The verdict is in on gun blue ... I tried it ... I like it!
BEFORE GUN BLUE, CLEANED UP WITH DREMEL BRUSH:
AFTER GUN BLUE, AS DESCRIBED ABOVE: