Use the brush attachment of your vacuum cleaner. It looks something like this:

I've tried the brush attachment. It did help some, but not as much as I'd like. Thanks for the tip, though.

I live on a dirt road also so I KWYM. The best thing for regular dusting is a microfiber cloth, kind of bunched together so it gets between the slats.

I'm awful, I keep forgetting to clean it until I notice it's partially brown. lol.

Since moisture probably gathers there from the return air due to high humidity, about the only thing to do is wash it with 409. 409 is my friend. :)

If that doesn't work, they can easily be painted. Good luck!

I'm on a crushed limestone road, in an agricultural area that is drought-stricken. The only thing that works for me is taking the grills out and hosing them off. If I just cannot face doing that, I use the brush attachment on this Metro Data-Vac vacuum cleaner: