I have had pretty good results using the "Rust Reformer" on a lot of parts, some I just left uncoated and other stuff topcoated. It does require the wire brush clean-up that has been mentioned before any coating is applied, mainly just to get rid of the loose flakey stuff. I guess it just depends on the need and application if a person should sandblast and start fresh.
On Al's wire fencing I imagine the right track would have been the brush on stuff since 90% of the spray was wasted!
The Penetrol deal is interesting. I never thought about using it for a prep-coat but it has been a valuable additive for me since the early 1980's. In my sign painting days it helped the One Shot lettering paints flow much better, it improved coverage and toughened up the finished product. I even use it to preserve fading and oxidizing painted surfaces and over rust!
My old '55 GMC is a rust bucket but 99% of the people that comment on it say don't paint it and I really don't want to put that much time and money into it either. It would oxidize very fast after cleaning and it was pretty difficult to rub it back out. A coat of Penetrol just wiped on with a rag like stain has saved a lot of work keeping the old beater a little more presentable.
The pic is the difference with Penetrol (center area with GMC logo) and without and a pic of the can. It doesn't stay that shiny which is what I wanted.
Oh and the last pic is an old pistol my Mom dug out of the ground after getting a hit with her metal detector nearly 50 years ago. She soaked it in Naval Jelly and got this result. Unfortunately there's no before pic but it was basically a ball of rust in the shape of a pistol. I have never re-cleaned it besides just dusting it off.