S&W did not rate any .38 Special J-Frames for +P ammunition until the introduction of the "Magnum" J-Frame in 1999. While pre-1999 steel J-Frames such as the Models 36, 40, and 49 will handle limited amounts of +P ammo, airweights such as the Models 37, 38, and 42 should be fed a diet of standard pressure ammo only. The issue is that the aluminum frame is not as strong as the steel one (which was marginal for +P in the first place) and thus can and will stretch if fed too much +P ammo.
At most, I would recommend firing only a cylinder or two full of +P ammo to check POA/POI and crimp-jumping in a carry load and then using standard pressure for everything but carry. Better yet would be to get a good standard pressure carry load like Hornady's 110gr Critical Defense and avoid +P all together.
As for the finish, you can touch it up with something like the Birchwood Casey product if you like, but I doubt it will last long. Aluminum cannot be blued like steel can and, as such, most guns made of it have some sort of paint or other coating applied as a finish. As such, the finish on aluminum-frame guns generally isn't very durable and they will show holster/carry wear sooner than their steel-frame counterparts do. Even with modern J-Frame airweights, the silver-colored ones like the 642 are more popular than the black colored ones like the 442 because if/when the finish wears, it's less noticeable. Unfortunately, finish wear on aluminum is just the nature of the beast and part of the trade-off for a lighter weight gun.
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