I have had the same issues with contamination. I remove the air adjustment and the aluminum piece (with the three sharp prongs) inside the gun that charges the powder and blow everything out. I also hook air up and pull the trigger and shake the gun, and I'll still get powder out. Ever since I started removing the aluminum piece, I have not had any issues with contamination. A powder coating group I belong to had a post where someone had made a modification to the aluminum piece, which he said elliminated "burping." Basically he tapered the back of the piece to elliminate the flat surface. I would imagine it would also help in getting the gun clean. I'll try to attach a picture.
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