This is true. And yes the primary filter can be blown out but there is the risk of the air ruining the filter so you have to be careful. (dont blow out the secondary) As long as my filter in my UMP goes before it gets dirty and as cheap as they are I just replace it.That "inner" filter is only meant to keep your intake from getting anything in the intake while changing the main air filter from what I have been told.
Bill What number from which manufacturer do you order for the DF for $18, that sounds good?For 18 bucks CDN, I can buy a new Donaldson filter for my DFR intake...and this filter is probably twice the size of an UMP's filter. For piece of mind, I just replace it regularly whether it needs it or not.
I bought my Donaldson with mount for my Rhuggy build at Ironman in Corona, CA. It cost me $127.00 or so and I picked up. Can't imagine shipping to be more than $20 or so anywhere in the country.