Well, now having experience in using RP-342 Cosmoline, as well as fluid film, they both have their place. Bottom line, Cosmoline out performs Fluid film by a long shot. It isn't like tradition undercoatings, that are really bad or worse than just leaving it alone and paint only. Cosmoline will completely seal out moisture and oxygen, permanently...
Now I said both have thier place. Cosmoline is more permanent, it won't wash off, can't even power wash it off. You have to use some sort of solvent to get it off. So in an area that you can't easily reach, that maybe later you want to remove it, I wouldn't use there, and Fluid film may be better, as you can easily rinse it away, inside door panels, body panels, etc. It is perfect for anything under body. It drys semi hard, and doesnt attract dirt, looks clean and perfect for a very very long time and is self healing. Moisture can NOT get under it, it will NOT peal, and once it's on, will never rust there again. It feels like paint in a way, and is not sticky.
I had planned on cleaning all the rust from my frame, but found it was nearly impossible to remove it all without taking the body off the frame. I couldn't do it before winter, so I power washed everything I couldn't reach to remove the rust. Removed most of the scale I could, and said screw it until I can pull the body. Sprayed it over the rust just to stop it for the winter, or at least slow it... You can still see the rust through the amber colored spray, but it has never rusted again, or formed more under it. It literally soaked and wicked into the rust and stopped oxygen and moisture from entering and stopped it... I am a soldid believer in it now... Best thing is, it's 133 bucks for a case of 12 aerosol cans, and I used maybe 6 cans to do it all... Cosmolinedirect.com... Frame looks like new still... They sell black rp342 as well, I ended up doing my axles in it instead of painting them, and looks just like my frame that I painted before coating. Wish I had skipped paint and just used the Black.
I learned about this after researching fluid film annally, and found a few guys that did this in the upper north eastern states, where I'm from, and drive them daily in the Salt and snow, and their underbody and frame look like new, with no maintenance from several years before. Fluid film does not, is not as good as this, bottom line, no if ands or buts.. The only maintenance I've done is where is change my fuel filters on the inner frame rail and the diesel dissolved it a bit. Just sprayed over it to make sure it was good to go.
But like I said, fluid film has its place as well as Cos, and work well together in different applications. Just have to think about it, Cosmoline is more permanently applied and fluid film is more easily applied in areas hard to reach, like bed rails, inside holes etc. ...
