NOT A PAINTER
I have been powder coating for about 20 years now, and I have done a lot of valve covers, but in the past, I have done them with paint.
Here is how I would do it: Blast them down to bare aluminum. I use medium grit crushed glass. I have found that it leaves a perfect surface for paint or powder coating; not too rough and not too smooth. Then I would use the Rustoleum Automobile rattle can primer that @RustyRatRod mentioned. It is lacquer based, it fills blemished very well and it is easy to sand. I get it at Wal Mart for about $7 a can. It is pretty much the same as the lacquer base Duplicolor primer sold at car parts stores but a lot cheaper. I'd spray a light coat followed by two medium wet coats. I always wait at least 10 minutes between coats to give the solvents time to evaporate. Then I wait a few days to make absolutely sure all of the solvents have evaporated. Then I would wet sand the entire valve cover with 320, 400 and 600. Then, using 320 wet and a sanding block, I would sand all the primer off of the top of the fins, letters and that big rectangle in a straight linear fashion from end to end. Do not sand these places in a circle or side to side. If you do want paint on that rectangle, then sand it with 320, 400 and 600 like the rest of the valve cover. I would use a scotch brite pad to get in tight places like in between fins. You can use wet sandpaper that has been folded over several times to make it about 1/16 - 1/8" thick. Then paint them. Always make the first coat a light coat. Do NOT go for full coverage with the first coat. That is an invitation for runs. Then you can go with medium wet coats. Then I would let the paint dry for at least two days before sanding the tops of the fins etc. I would do that with 320 grit wet sandpaper on a sanding block going in straight lines end to end like mentioned before. By waiting several days and wet sanding in straight, end to end lines, I can pretty much guarantee you it will work out fine.
BTW, here is a pair of VCs I did for my Barracuda with powder.
Totally agree with hand sanding. I did not see where he said he used a DA. Maybe I looked over it. But you are RIGHT! DA is NOT the way to go on a project like this.
I have never used self-etching primer. Every whole car paint job I have ever done was stripped to bare metal, bare metal scuffed with 80 grit on a DA and then a few coats of epoxy primer. Further coats of primer stick to scuffed/sanded primer just fine.
I have been powder coating for about 20 years now, and I have done a lot of valve covers, but in the past, I have done them with paint.
Here is how I would do it: Blast them down to bare aluminum. I use medium grit crushed glass. I have found that it leaves a perfect surface for paint or powder coating; not too rough and not too smooth. Then I would use the Rustoleum Automobile rattle can primer that @RustyRatRod mentioned. It is lacquer based, it fills blemished very well and it is easy to sand. I get it at Wal Mart for about $7 a can. It is pretty much the same as the lacquer base Duplicolor primer sold at car parts stores but a lot cheaper. I'd spray a light coat followed by two medium wet coats. I always wait at least 10 minutes between coats to give the solvents time to evaporate. Then I wait a few days to make absolutely sure all of the solvents have evaporated. Then I would wet sand the entire valve cover with 320, 400 and 600. Then, using 320 wet and a sanding block, I would sand all the primer off of the top of the fins, letters and that big rectangle in a straight linear fashion from end to end. Do not sand these places in a circle or side to side. If you do want paint on that rectangle, then sand it with 320, 400 and 600 like the rest of the valve cover. I would use a scotch brite pad to get in tight places like in between fins. You can use wet sandpaper that has been folded over several times to make it about 1/16 - 1/8" thick. Then paint them. Always make the first coat a light coat. Do NOT go for full coverage with the first coat. That is an invitation for runs. Then you can go with medium wet coats. Then I would let the paint dry for at least two days before sanding the tops of the fins etc. I would do that with 320 grit wet sandpaper on a sanding block going in straight lines end to end like mentioned before. By waiting several days and wet sanding in straight, end to end lines, I can pretty much guarantee you it will work out fine.
BTW, here is a pair of VCs I did for my Barracuda with powder.