[Shop-talk] setting up compressed air lines

[Obaa]

I know this has been extensively covered before, and I've already done
searches, 
so I promise this is something new... :)
I'm planning on running
compressed air lines throughout my garage during the 
next few weeks.  I will
be using black pipe because copper is currently too 
pricey.  I've seen the TP
Tools pdf on routing lines and drops, so have that 
covered. My questions are
probably so basic that they're assumed knowledge....
 
I've read that rust is
one of the main disadvantages to steel pipe.  Is there a 
paint/coating I can
run through the pipes before installation to keep down the 
rust?  I've also
had problems before in trimming pipe to the correct length and 
sealing the
joints in the past.  I'd like to keep the pipes
snug to the walls, so
wondering if there was some formula/trick to determining 
where to cut a pipe
so the length ends up correct, and if there is a thread 
sealant so that the
pipe will be properly sealed in case it was cut slightly 
"short"?