1964-1974 [Subscribe to Daily Digest] |
Ahhh, at last a subject on which I can feel I really *know* something! That's because I started out with a Sonett consisting almost entirely of rust (for a while I thought the Swedes actually had devised a process for casting the chassis out of iron oxide) so I have had a LOT of opportunity for experimentation! Here's my corrosive take on the situation:
-- Why Rust Forms in the First Place: Bare steel in contact with oxygen eventually will oxidize (rust.) Oxygen from the air will work, oxygen from water works faster. Paint, undercoatings, etc. deter rust because they keep the oxygen from coming into contact with the steel -- until the paint surface is damaged or becomes porous. Rust doesn't actually "spread like cancer" -- what happens is that the rusted area expands as it oxidizes, breaking up and lifting the surrounding paint and metal, allowing more oxygen to get in, which expands and lifts more surface, allowing more oxygen in, etc., etc.
Knowing this suggests three ways to deal with:
-- Prevent It In the First Place: If you've cut out the rust and replaced it with clean new metal, your best bet is to protect that metal with a good paint job. As others have said, good prep work is your best guarantee of getting the paint to really stick. I don't think it even matters exactly what type of paint you use as long as it's compatible with the surrounding materials and will adhere firmly to the metal. You've also got to keep an eye on the paint and touch it up when damaged -- most of the rust that's been my fault has been because I was too lazy to fix small nicks, rock chips, etc., allowing rust to start the paint-lifting process described above.
(That experience leaves me unsure where to stand on the truck-bed-liner stuff, aftermarket undercoatings, etc. On the one hand, they might toughen the surface and reduce the risk of the paint getting chipped; on the other hand, they'd make it harder to spot damage. Maybe the sensible decision is that if you ARE going to keep an eye on the paint and fix as necessary, leave the paint bare so you can see it; if you know you're going to be too lazy to do that, maybe the coatings would buy you some time before you get a ping that's bad enough to start rust.)
-- Neutralize It So It Won't Spread: If you can't or won't cut out the rusty area, there are several chemicals that stabilize the rust so at least it won't continue to expand and admit more oxygen to the metal underneath. We had a chemist give us a rundown on these once; basically, they all contain something along the lines of phosphoric acid to etch into the rust, and then something that reacts with it and converts it to magnetite, an iron derivative that's more stable than iron oxide (rust.) That's what Duro Extend and similar products do; Eastwood's Corroless stuff works similarly but includes glass flakes that supposedly seal over the magnetite and protect it from abrasion, UV light, etc. (With Extend you have to paint over the treated area if it's going to be exposed to sunlight or it'll flake off; on the other hand, the front springs I treated with it still look fine as they don't get a lot of sunshine under the fenders!)
One key thing to remember about all these products is that you do NOT want to apply them to clean metal -- the phosphoric acid will etch the metal and actually cause it to rust before sealing it. (It says this right on the can of the Eastwood stuff.) So, painting new metal with one of these products does NOT seem like a good idea. On the other hand, I've had good luck with both Corroless and Extend on components with surface rust -- they do seem to keep the rust from developing any further, and paint will stick better to these treated surfaces than it would if you just sanded the rusty areas smooth and painted over them with Rust-Oleum primer, for example.
Another thing to remember is that the whole reason we want to get rid of rust is that it's not nearly as strong as steel -- so you only want to use this stuff on "cosmetic" rust that hasn't gotten bad enough to reduce the strength of the metal. If a structural part of your car has rusted badly enough to weaken it, the treatment may stop it from getting any rustier -- but it won't put back the strength that has already rusted away!
-- Seal It Off So It Won't Get Worse: One category of product I have NOT tried is the POR-15 type of thing, which as best I can figure out from the ads is a type of sealant rather than a chemical "neutralizing" treatment like Corroless or Extend. In other words, the goal seems to be to hold back the rust by hermetically sealing it away from the oxygen, rather than by converting it to a more stable metal -- it works more like a "super paint" than a chemical conversion. This seems like potentially a sensible approach in a situation where you've got a mix of rusty spots and sound metal -- you won't rust the sound metal the way you would if you just applied Corroless or Extend over the whole thing, and you'll be doing a better job of sealing off the rust than if you just painted over it. Maybe I'm wrong about how this stuff works, and if so someone who has more experience with it can enlighten me, but that's how it seems to me.
Again, you'd want to keep in mind that prep is critical, and just sealing up the rust won't make the metal any stronger, so this is still not a substitute for cutting out and replacing a rust-weakened structural part.
-- What I've Done to My Own Personal Rustbucket:
Cosmetically-rusted bits that either don't need to be or can't conveniently be replaced -- such as suspension parts, bumper mounts, etc. -- have gotten treated with Corroless or Extend and then painted. They're not getting any rustier and look reasonably nice.
Rust-damaged structure has been cut away and replaced with new patches welded in. This new metal I've treated two ways:
1) Semi-protected areas such as the sides of the trunk, inner sills, etc., just got primered and painted, so I can keep an eagle eye out for any paint damage and nip it in the bud.
2) Areas such as the outer floor, inner fenders, etc. that unavoidably are going to get blasted by gravel, road debris etc. got primered, painted, and then sprayed with rubberized undercoating. I don't figure the undercoating is going to last forever and don't really want it to dry hard and possibly separate from the paint -- all it's supposed to do is absorb some of the impact of crud hitting the car so maybe the paint underneath doesn't get chipped so easily.
So, this may not be the perfect strategy, but at least it IS a strategy... admittedly my car was never concours to begin with and if it were I couldn't expect it to stay that way considering that I drive the living whee out of it -- but so far it seems to be working as best I can tell...
posted by 68.13.18...
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