chemical bath for re-used C filters

I reuse my old carbon filters as a first stop for larger algae in my emperor HOB units.  I cut out the bottom then remove all the carbon.  I spray them down-w/ non RODI-unfortunately-then let then dry.  I replace them a few times before replacing the actual carbon filter.  It keeps it cleaner (the carbon filter) longer.  I can see brown residue behind the plastic backing.  I was wondering if there was a chemical soak I should use in order to completely eliminate this residue B4 putting it back in my tank.  Or maybe I should just cut out the entire back of the filter.
Any help would B appreciated.