I've read a lot on this Prime All stuff and have seen a lot of people fail to make good percussion caps, mainly because the compound falls out.
I have discovered the secret, that I stole from someone online, and tested it today.
The secret is Duco Cement!
https://www.amazon.com/dp/B0000A605H...=&sprefix=duco
Pour a 1oz tube into 10oz of acetone and your caps will be good and hard. I fired 25 caps today and they all went boom. The redish crud came right off so don't worry about that. They are still a pain in the booty to make but will work reliably.
I also used some coper strips instead of soda cans and they worked just as good. .004 or .002 will work. I tried .008 and its a no go in the die.