I used Tectyl. The stuff I got was in a 4L tin and quite thick, so I thinned it out a bit with turps. Then filled a parts washer gun, parked the ute on the grass behind the shed and sprayed/fogged the inside of every panel I could get to, as well as down the inside of the chassis rails. You can see the oil fog rolling out the holes and drains, so I assume that there some coverage in there. Either way, it is better than what was there before I started.
I used the spray cans of Tectyl in the past. They do an OK job, but only when you can point the nozzle at it directly. It is a bit difficult to get the right angle on the nozzle so the stuff goes where you want it. I was doing a Suzuki Sierra, so pulling door trims off wasn't that hard, but on other vehicles it is a pain.
I know a guy that used a 1 and a bit meter length of that 5mm micro irrigation tube with the little plastic sprayers on the end. It was attached to the washer gun and fed up inside channels and the body work. His reasoning was that if a plastic sprayer fell off inside a panel it doesn't matter. They are not big enough to rattle and won't induce corrosion. He also restored Alfas, and if you didn't do something they would rust before your eyes.
The important thing is to get the stuff into the seams as that is where the water and gunk gets trapped and the rust generally develops.
I used one of these jiggers.
http://www.supercheapauto.com.au/online-store/products/Blackridge-Air-Engine-Cleaning-Gun-750mL.aspx?pid=156475#Recommendations
