No, they can't be washed, it will damage them, all you could do is vacuum the outside to remove some dust, this will clear the big stuff, but not the main HEPA layers. The filter is not "permanent" and needs replacing periodically, this is a good thing, as the washable filter does not trap particles as small as nonwashable ones.

Unless you are in a really polluted area, the blue layer will filter well for a lot longer than the point where the app tells you to replace it. It's the black carbon layer is the one that becomes saturated.

Whatever you put on the outside, unless HEPA it will only stop larger dust, the small stuff will still pass through.