I used muriatic acid mixed 50/50 with water and a toothbrush. Worked well for me but you have to be careful with the acid. There's the safety precautions obviously but it seems different suppliers products react differently. Test it on the backside first. I almost forgot, do a small area at a time and have a water hose handy. You only want to leave it on the aluminum for about 30 seconds and then flush thoroughly. Repeat as required