No Mixing involved! You've got the best possible solution for getting all that oxidation off of your doors.  Add a 1/4 cup of Sand & Scrub onto a Medium Etch & Clean Brush , then scrub it into the door and rinse it away.  One gallon does approximately 50 8' x 8' doors.

Note: All surfaces need to be thoroughly de-glossed. While Sand & Scrub has abrasive characteristics, It's use alone is not a substitute for making sure that the proper gloss profile reduction on a SMP door is achieved.  Additional mechanical abrasion (Scotch brite pad or sandpaper for instance) may also be needed to insure that the proper de-glossing profile is achieved. It is up to the painter to inspect and make sure surfaces are properly de-glossed prior to painting.