You don't have to sand mandrels, just a good soap (degreaser) and water wash and really dry before you dip them. Might just be the release, too. If it is anything like what I got in my starter kit... Uggg. When I ran out of that junk and had to buy a new bottle, I was so SHOCKED to see that it didn't flake off when I looked at it the wrong way.
Ahhh.... my Fireworks days....
I soooo hated that torch. What a piece of shit.