Greg, it all depends on how bad or extensive the thinning blue is. I felt the advice you got in your other cold blue thread was pretty good... especially the recommendations to use Brownell's Oxpho Blue.
Cold blue is no substitute for a good complete rebluing job. It can be difficult to get even coverage, especially on larger areas. But of the cold blues I've tried, Oxpho Blue is the best by far. It is also the most durable.
It does take a bit of practice to do a really good job of getting it to take evenly, and I'd recommend trying it first on a cheap gun or junk set of barrels to refine your technique. There are a lot of tips on using it in the Brownell' Gunsmith Kinks books. Always pour a small amount into a bottle cap or something and never return your application swab or used solution to the bottle . I've always used the liquid, so can't comment on the creme.
One technique that has worked very well for me is to use a small wad of 0000 steel wool as an applicator pad after using a cotton patch for the initial coats. I use the steel wool dampened with the Oxpho, and rub it into the surface as hard as I can to get it to blend well with the surrounding blue. Vary the pressure of this burnishing to vary the result. It is that tough that the steel wool won't remove it, while most other cold blues would just get rubbed off.. You keep applying it until you are satisfied with the color, or until it won't take any more.
I've done some spot repairs that were intended as temporary. But they turned out so well that I had trouble finding them after a couple years.... even though I did the work and knew where to look. And a couple of those involved removing spots of rust and using emery cloth to remove small pitting. Of course, care must be taken to polish the metal to the same level of polish as the surrounding metal, or it would stick out under scrutiny.
