You can buy them in what they call the "6-pack" -- 6 small cans. You can do as Ryan says or open up the can but just be careful opening and closing, and cleaning out the can's groove. That stuff is on par with JB Weld. Try to not get any of that crap on you. Tough to get out. Also, it might appear to stick to the metal and adhere but I have had it where it was prep'd per the instructions and the POR-15 just peeled right off.

Yep, I was thinking of Tannic acid for rust conversion.

https://www.theruststore.com/Rust-Converter-FAQs-W48.aspx

Rust Converter FAQ's

What is Rust Converter? Rust Converter, a water-based primer, contains two active ingredients: Tannic acid and an organic polymer. The first ingredient, tannic acid, reacts with iron oxide (rust) and chemically converts it to iron tannate, a dark-colored stable material. Tannins are a group of water- and alcohol-soluble natural products extracted from fruits, trees and grasses. The second active ingredient, 2-Butoxyethanol, is an organic polymer that provides a protective primer layer. The overall chemical reaction converts rust into a stable, black protective polymeric coating that serves as an excellent primer for both oil and epoxy based paints.

The only way to stop rust is to keep water away from it. Rust attracts/holds moisture Water is a constant source of oxygen. Steel must be encapsulated from oxygen to not rust.

Bondo and spray paint does not stop moisture from passing through it, despite what most people think.


Remove that trim piece and chip out all the rust you can, then wire brush it. If a hole has rusted through into the door, take the door card inside off and look inside the door and see what's going on in there. Water always gets into the door and is supposed to drain out a few small holes along the bottom of the door. If you don't have a bunch of crap clogging the drains in the door consider yourself lucky.



Anyway, start by removing that trim and cleaning everything up to see what you're really looking at. When you're down to metal with just a tiny amount of surface rust, and no drain problems with the door itself, then you are in a position to stop it.



There are several ways to treat and encapsulate the metal after that. IMO phosphoric acid (not the milky stuff) is good prep, POR15 is good on rough surfaces (it basically like polyurethane plastic) but it must be covered with something. I use thick epoxy putty to fill any holes - no very sandable but it's awesome. Paint over that.


Fluid film (and things like it) are for areas that can't really be accessed or are not worth painting - like underside parts or the inside of panels, or like inside you're door after it's cleaned out and maybe dowsed in a rust treatment. It's basically saying F U to rust but not caring what it looks like. The best ones are cosmoline or sp-400, nasty stuff. The liquid can be painted on with a brush, easy. If you get the little spray can version, have a respirator- that mist can suffocate you..

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1985 300TD 1981 Scirocco 1.6D conv 1986 Golf 1.6D 2003 Golf TDI

Yup, you're right, it technically is a rust converter.

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1998 Ford Escort ZX2 5 speed - 279,000 miles My Daily

1992 Mercedes 300D 2.5 202,000 - Pure junk
2000 Mercedes E320 Black - 136,000 miles - Needs repair

Don't forget to grease the screw and threads on the spring compressor.