Of course something like this dipping would be the best method
but what I have seen done that may be an alternative if on a budget.
Shutz guns (aka - undercoating guns) are available with wand kits.
Whole setup May be $40-75. depending on kit.
I have seen shops shoot acid into the frame and let set about 5-10 minutes than flush with a hose and at that point when blowed out and dry, shoot whatever you want through the undercoating gun from epoxy to por or rubberized undercoating.
Of couses this is allways hit or miss when you can't see what your doing.
Some wand kits come with side holes on end this will get you in the tight places with the acid and last coating applied.