Rust
Coating: A "coating" is anything designed to do nothing more than protect the bare steel from being exposed to elements. Sometimes just a layer of paint, sometimes a “rubberizer undercoating”, etc. Either way, just a coating, usually black, the purpose of which is to protect the steel from the elements. Better than nothing, and can do some good if used from day one (before any rust develops), and if regularly maintained. Drawbacks: if not applied correctly, or if not maintained, can allow moisture to creep in past the barrier, and then trap that moisture inside. Then it grows unnoticed. Furthermore, if rust has already developed, a 'coating’ will do nothing to treat the existing rust or stop it from growing underneath the coating. So, perhaps used best/only as a short-term preventative on some perfectly rust-free steel.
Converting: Anything designed to convert existing rust through a permanent, chemical reaction. Sometimes advertised for preparing a rusted surface to be painted, or for simply stopping rust in its tracks. Eastwood’s products are good examples. Their “Rust Converter” product is designed to ‘convert’ areas with heavier amounts of rust. Their “Rust Encapsulator” is designed for areas of lighter, surface-rust, or for areas that you have been able to scrub and prep pretty well. Their “Internal Frame Coating”, is a bit of a combination product: part converter, part encapsulator. It’s intended to be an all-in-one product for treating and sealing the inside of rusty frames, where you can’t easily prep the surfaces or check in on it regularly. Finally, POR-15 also belongs in this category. It is perhaps best compared to Eastwood’s Rust Encapsulator. One main difference is in the curing process (waters speeds it up for POR-15), and also in the prep-work (POR-15 requires more, and is reportedly difficult to work with).
Creeping/Healing Seals: Anything designed to spread after application. Similarly, if ever rubbed away
in small amounts, the surrounding area "self-heals" by creeping back over the bare areas. Fluid Film has an active ingredient of lanolin. It creates an oily/waxy moisture barrier that will prevent any water from reaching the steel it protects. It never quite dries, and remains somewhat sticky/tacky for however long it stays on the surface. But this also means it's not permanent. How long it stays on the surface is determined by how often it gets pulled through mud, rubbed against rocks, pressure-washed, etc. So, must be re-applied on some kind of regular basis. Eastwood also offers a product “Heavy-Duty Anti-Rust”, which fits this category. And finally, New Hampshire Oil Undercoating (NHOU) seems to be another, very similar product.
Notes:
- Others' opinions are here and here.
- Buy a scope to check the inside of the frame. Buy a sewer jetter to clean it out.
- Consider drilling 1/2" holes in the frame, about an inch inside each control arm mount (like this).
- Good discussion (with pics) on applying Fluid Film here. Don't forget to tape the holes.
- Discussion on removing Fluid Film, to apply Eastwood, here.