You CAN do all kinds of things to make them LOOK like new. The question is, are they LIKE new?
The filtering is VERY delicate and can easily be damaged. Blowing a filter out with an air hose is commonly done, but is a very bad idea if you still want it to actually filter dirt. The air pressure widens the passages/holes significantly - in fact this was a standard trick in production class racing - blow the hell out of the filter to pass more air and allow a bigger main jet. BUT that was road racing - wouldn't dream of doing it on a dirt bike.
I'd want to see a NEW filter and a washed filter under a microscope at the very least before I was happy about it. Flow testing and filtration testing would be nice, lol!
I DO like washable foam filters.
There is a big thread on filters somewhere, search on K&N.
Seek, and ye shall find.
Inspiring, Informing and Connecting travellers since 1997!