For enclosed areas you can get a rocker shutz gun and modify a tube similar to a under coating kit. These have a spray end attached to spray a fan. Just slide it in a access hole and pull back out as you spray.
A good auto body suppy store should have a inexpensive under coat spray kit. You could also fabricate a end to do this also.
Dave
That's pretty good to know Dave. Never thought of anything like that
Another good thing that I found works just as good is to Completely remove your trunk floor, and any other panel that may be in your way, ( even if they're not rusty, take em out anyways. It will save you a ton of time in the long run
..........J/K
But it was pretty convenient for me
Actually, I'd just listen to Dave if I was you