i have also heard that you can combine smoothcast 321&320, i think 320 is stronger but i'm not postive. I also heard to brush 321 on the outside and 320 on the inside or vice-versa. like i said i saw i'm not positive about the order but the product's do work. in the vid i watched a while back he said you can even do it indoors(workshop etc).
http://www.smooth-on.com/ hope i helped a bit, also this stuff is brushable for the outside, and slushable in the inside
p.s always use proper safety precautions