A bit of cotton-wool pushed down the spout of a funnel is ideal for filtering home-mixed chemicals and would, at least, remove that possibility. Seeing as you mentioned that you use liquid-concentrate based chems then the possibility of something in the liquids goes to the wash water. You can use a household water filter jug to clean your rinse water very successfully.
I did always wonder though, how solid particles could possibly stick firmly to film, instead of just swishing past in the rinse . . .