I was unaware of cerrabend but it sounds like a good investment
if you plan on bending a lot of pipes. Another suggestion would
be fill the pipe with pea gravel. I've never used it but pea gravel
should have much more staying power than sand.

       I've noticed that pipes via muffler shops tend to be 'tail pipe'
quality
for both exhaust pipes and tail pipes. Since I've never actually worked with
the benders and bendable pipes, I have to rely on what I've seen in the
bending and mentally recalling how long pipes, both tail- and exhaust-pipes
(and mufflers) last.  The distinguishing difference between tail and exhaust
pipes I am using is as follows: tail pipes are those extending from muffler
outlet to its exit from under the car; and exhaust means those pipes
extending from engine to inlet of muffler.
   I've also noticed the muff. shops tend to use the same tubing for both.
(If I'm wrong, then pls. correct me.)  That tubing seems to be thinner than
oem tail pipes and MUCH thinner than oem exhaust pipes.  As I see it,
exhaust pipes need to be thicker to prevent premature burnout; and tail
pipes can be somewhat thinner.  This due to exhaust handling much hotter
exhaust gases than the further-back tail pipes.
   That said, the thinner the walls of the tubing--all else being
equal--the easier it is to bend.  So, if their hydraulic benders are as
strained as they seem with thinner tubing, it's no wonder you and your
bender were taxed on a 'real' piece of tubing.  Time to practice on a piece
of the 'easy' stuff and see what it does.  s