Question about rust prevention: do you absolutely HAVE TO remove the rust first..? Can you get away with leaving what's there and then adding a coat?

[lizpat]

So, that's the basic question: must you absolutely remove the rust already there before applying any type of inhibitor or coating of some kind?

Is there not an easier way? Can you prevent further rust by simply cleaning the area and applying a coating that SEALS the area? The idea is that if you seal the area, you're preventing oxygen from further reacting with the metal.

The dominant view is that rust must be removed first, it just seems like a lot of work..

 

[Hall]

Once rust starts, it doesn't stop. Do it the "lot of work" way or just do the work later.

 

[AandPDan]

I was alway taught to remove ALL the rust.
Rust contains oxide. It contains oxygen.

 

[wings&wheels]

I think that is the intention of POR, though I have never used it. IMHO, minute levels of oxygen will remain and never stop rusting.

I have had great experiences with certain rust converters over a fine, well adhered layer of remaining rust, especially Corroseal. Not so great luck with other more widely available ones.

So, my answer to your question is; No...but choose your coating carefully.

 

[eljefino]

In theory POR-15 has you "convert" the rust from something bad into something good.

While the resulting metal hard, it's not ductile, and not as perfect as original.

If we could easily stop iron from rusting, the world would be a different place. The Golden Gate Bridge has a permanent crew of painters who work their way across and start over. Shipyards spend all sorts of money needle gunning rust then putting on dozens of layers of carefully selected expensive paint.

 

[Graham Piccinini]

That rust will come right through the new coats of anything you apply.

 

[demarpaint]

You can slow it down, a little, a very little by leaving it and coating it. The right way is to remove it, prime it and re-coat it. Rust converters are OK but I found not as good as removal, priming and re-coating the surface. Slapping a coating over rust is a short cut waste of time imo, and the rust will only spread and get worse.

 

[kschachn]

I was alway taught to remove ALL the rust.
Rust contains oxide. It contains oxygen.

Yes but the oxygen atoms certainly aren't going to come loose from the iron atoms, oxides are very stable. The problem with iron oxide is that it is less dense than iron so it expands and flakes off which exposes a fresh surface to more oxidation. All it takes is one rock chip or scratch to breach the surface and it will take off from there regardless of what coating or converter was used.

 

[Snagglefoot]

In the case of chassis rust beneath the car, you might as well spray it with your favourite oil type inhibiter to help slow it down.

 

[eljefino]

I was alway taught to remove ALL the rust.
Rust contains oxide. It contains oxygen.

Rust is a slow-burning fire. To put it out, you have to get all the "embers." Then it'll spontaneously re-combust anyway. It was born iron oxide and wants to get back to its natural state.

 

[UncleDave]

Think about it like tooth decay, if you dont remove it before you cover it up, you've made the problem worse.

 

[JoelB]

Just shoot it with fluid film if you don't want to do the work. Better than doing nothing, but not as good as removing and treating it properly.

 

[vavavroom]

Rust is hygroscopic. That means rust holds moisture and must be removed to prevent further corrosion. Could you displace the moisture in rust? Maybe mostly. Look into phosphoric acid, linseed oil and fish oil based primer. Also check out rebar epoxy paint.

 

[Kestas]

Not only does it hold moisture (free water) but it often is in hydrate form. This is your red rust. Without hydrate it is black. Because rust is not an adherent oxide it also has some porosity. All this will wreak havoc with any topcoat.

That's why I bought a sandblaster for restoration work. It can remove all rust down to the bare metal.

Otherwise, it's okay to put a layer of rustproofing over the rust. It'll slow it down considerably and it'll isolate the surface from the environment.

 

[JC1]

On the civic there was rust underneath the plastic wheel well covers. Rust proofing place never did anything with this. The first year or two that I started doing it myself, I never treated the rust, just recoated. Finally one year I cleaned it up with a wire wheel on a drill and then put some rust converter and then primed it before spraying on the fluid film. It take some time, but I want this car to last.

 

[doublebase]

You’re just not going to win with rust once it has started. Prevention is one thing but actual rust, a lot of it, is nearly an impossible and time consuming battle.

I have sprayed over rust with Fluid Film (and I’ve seen that done many times). It does seem to slow it down. Personally I’d just do this because even if you do “remove it” you really don’t remove it. You remove some of it, but it’s still there living in that metal and it’ll grow right back sooner or later. I know several body shop guys that will no longer even touch rust - flat out tells the customers, no - because it just comes back and it makes them look bad. It’s bad for business to “fix” rust and then a year and a half later it’s back bubbling through the paint. Gives them a bad reputation and then they’re doing it over again at a discount or for free.

 

[atikovi]

Can you prevent further rust by simply cleaning the area and applying a coating that SEALS the area? The idea is that if you seal the area, you're preventing oxygen from further reacting with the metal.

Theoretically, paint would do that, but I understand that microscopically, it's porous, so won't seal completely. Rust converters like POR15 are just less porous paint and can be pretty effective in that goal, but not perfect.

 

[wings&wheels]

If you do go the rust converter route, again buy a good one, but also follow the directions exactly. They seem to be a bit finicky and any trace of oil seems to stop it from working. I just replaced the battery on an old garden tractor. I had carefully sanded the battery support leaving a fine layer of rust, treated (Corroseal), primed and painted it maybe 5 years ago. Some of the topcoat came off with the battery pad, but otherwise, no new rust.

 

[Driz]

My 2000 Sierra is a classic example. I’ve been spraying the bed inner fenders with a mix of ATF / chainsaw bar oil/ petroleum jelly for a few years now. Clean it up well then sprat it with a schutz gun. Plug the door drains after washing them out and drying. Then pump in an inch or do and let it sit an hour or so then drain it. Spray some inside the rocker boxes if you can get at them . Thats often accessible via the drains or the sill covers.
It’s messy but works as well as anything . You’ll never be able to get at all the spaces and places that rust lurks to fix it so spraying with oil will work best once there is already rust present. No it’s not the real answer but there really isn’t one To be had. Used tranny oil is free, bar and chain $10/ gallon and Wallymart petroleum jelly $2 so it’s cheap. there’s youtube videos snowing how go go this process.

 

[porterdog]

I bought a 7 year old/80k mi Michigan CRV in 2011. Once I had it home I got underneath for an initial inspection. Pretty rusty, surface wise.

I sprayed it with CorrosionX once or twice a year until I sold it in 2017. In that time the rust advanced, but far far far more slowly than it would have otherwise. Sold the car at 175k and it still looked good.

I wouldn't use that method on a car that was intrinsically valuable, but for a driver, absolutely.