For the past 55 years or so I have used WD40 on the OUTSIDE of guns and tools .I use an old shaving brush that I had from my foot and wall locker display in the army. Just a short spritz and then use the brush to go over all the metal surfaces on the exterior of the gun's metal parts. Take pains to keep it off the wood and scopes. My understanding is that WD 40 is fish oil and the volatile carrier evaporates leaving a light non runny coating of oil. I never spray WD 40 into the interior of the action as it will gum it up. It's also a good idea to wipe your prints off the gun of a customer-you never know where your prints may show up! I buy Brake Kleen non chlorinated spray to clean the actions out, just don't use Brake Kleen on plastic parts-it is bad for plastic. Happy Holidays.Mel Chung-Gunsmith in Hawaii
