Hi there,
Interesting subject you raise here. Your desire to hold on an old patina as such reminds me of how fashionable it has become for a lot of electric guitar owners around the world who own old beat up, well used and abused guitars. The more dirty and beat up they look, the more appeal they are supposed to have. I myself am still coming to terms with that concept.
In any case, your desire to hang onto the old dirty,rusty, beat up look with your air cleaner, can be achieved. The one thing you must keep in mind is, a soon as you start working on it from where it is exactly now, you will change the look to a more improved one. For example, you could start by using a carnuba wax paste on the blue body. By doing that, you will start to clean up the blue color to some extent and give the old paint a kind of fresh, warm low gloss as well. Now that might be a look you don't want, as you will be enhancing the look of the old blue paint to some extent.
So my advice is, if you really like the air cleaner the way is now in the photo, an old trick is to just wash it down with cold water only , let the whole thing dry off super well, and then buy a rattle can of mat clear lacquer, and apply 2 or 3 light even coats over the whole thing, letting each coat dry off properly. This will protect, seal off and keep the part looking the way it is now, filth, rust, crud and all. It's like a frozen moment in time when you carry out this process. TIP: Don't apply the clear with too many wet coats, as the old paint may play up on you, and cause problems.
Hope that helps,
Greg.
