You
have to do the
sanding the day that you shoot epoxy. Either that or use a aluminum prep wash just before you epoxy, (you still need to sand with 80-180 just not the day of) as aluminum oxidizes almost immediately after sanding. If you are not experienced, sanding with 180 grit on the DA is safer (and perfectly acceptable) than 80 grit, especially if it's a softer temper aluminum such as 3003. Even if it's 6061, 180 would be a better choice IMO. One other big thing is that you do not want to do any type of work on steel while your aluminum is in bare metal. No grinding, no sparks around your aluminum bed. Any ferrous metal that comes in contact with the aluminum will cause corrosion which will result in paint failure shortly down the road. Clean your tools, use fresh sandpaper, take any necessary step to ensure that doesn't happen. If you have also done steel work in the vicinity of the aluminum or don't know, then sanding, wiping it down with a W&G remover like SPI 700, then resanding is a good idea to help ensure that you don't have any embedded steel particles in the aluminum. All it takes is one and the galvanic corrosion starts, which results in the paint bubbling and failing in that area. Even the OEM's have had trouble with this. OEM's now state that any aluminum repair and refinishing needs to take place in a seperate area from where steel is worked on. Basically a separate facility.
I know of 2 shops that are building/setting up separate shops just for their aluminum work.