Zagato wrote:

You will find your answer repeated a few times in this thread but quickly.... Any acid rust converter treatments (Ku-Rust, Fertan, Jenolite, RC900 etc etc are all the same about 6-9% acid content in various mediums) are only effective if you get the rust right down to good metal. They don,t soak into rust much at all. They are effective on metal which you can grind back to near good metal BUT on Land Rover chassis you cannot get machinery into the various nooks and crannies or in seams or welds so simply spraying it on top of rust is next to useless especially if there is significant rust. "Professionals" and DIYers have realised that ACF-50 stabilises rust more effectively, even after a year when it becomes inactive. I have had a test area going back 9 years that I have retreated once and it has not got any worse. I'm not suggesting this would be the same on a Defender chassis... how it Is prepped... seams, welds, conditions... all considered but it is definitely more effective than acid convertors in rustier areas. Don,t be tempted to use the tough hard coatings you are just trapping in rust, not stopping it... despite what so many products claim about stopping rust, they don't, they just stop it showing through or simply cover over rust!
ACF-50 is not that durable to road conditions long term, especially off-road so you need to put a covering over it to protect it. Dinitrol 4941 is ideal for this. At first it will have a tacky, wet look, but it will go off to the dry matt finish after a couple of weeks 👍 You can buy 4litres of ACF-50 for £80 from The Sportsbikeshop you will probably only need half of that. If using a compressor you will need the paint spraying type or brush it on, or spray it on with a simple garden hand pumped sprayer e.g. Old empty bug sprayer.
The point with Defender chassis is to have the ability to slow the rot right down, be able to keep an eye on corroded areas and retreat when necessary every 4-5 years if required.