Since it's too cold outside to do anything and the topic's come up, I wrote a howto on dealing with rust. If folks like it, please move to the howto section. I'm hoping folks will add to this with more pictures and techniques, it's a big topic.
Rust is ferric oxide and it is the enemy. The main thing to keep in mind is that whatever you do about rust, you're really just buying time--eventually all of our cars will rust away if they don't get recycled. So when thinking about dealing with rust, choose the best methods for delaying the inevitable as long as possible.
For information on rust and the chemical processes that produce it, see http://en.wikipedia.org/wiki/Rust
Also, as a disclaimer, I'm using lots of stuff from eastwood, so I reference them a lot. But I don't work for them--I just like their stuff, and I joined their buyer's club, so I get shipping for free. And while I've worked with metal a lot in my life, I'm new at car restoration, this is just the best information I've gathered so far, and YMMV.
Obviously, the best way to deal with rust is to avoid it all together:
- *Flush areas that collect dirt (and thus water and salt), especially the rockers (there's a plug at each end, you can remove that and flush the inside with clean water), the rear wheel wells (the bottom of the front and back has a shelf where dirt collects), and under the bumpers.
*Check for chips in paint or where the undercoating has failed under the car.
*Check seams and weather stripping for cracks and failures.
*If you use any body filler, make sure you seal it up front and back. For example, if you shave your antenna, back the hole with metal, otherwise moisture can work it's way into the filler from the back and start rust on the panel. If nothing else, use aluminum tape.
Places to look for rust:
- *Stress points, such as where the seat belts and seat rails mount to the floor, and the jack points.
*Wherever water collects or is supposed to drain: the bottom of the bay for the spare tire, the rockers, especially at the ends, under the bumpers, the bottom edges of the doors, around the seams at the gas filler, and, of course, the rear wheel arches.
*Under the exterior trim--it can get really bad before you see it there.
*Seams--the body is spot welded together and the seam sealer used gets brittle and cracks with time, and becomes a place that water gets trapped.
*Under hinges--rust can start underneath the door, hatch, and hood hinges.
*Behind panel repairs--weld through primer yields some protection, but not much really, so the backside of a panel weld is a place rust can start
*On Si's, the sunroof panel--when the weather stripping's adhesive fails, water collect in the weather stripping against the seam between the panel and the reinforcing frame underneath. Usually the first sign is bubbling paint along the edge of the sunroof. If your sunroof is clean, take it out and check under the weather stripping.
*On first gens, check the pan and the pan hard bar, both are prone to rust.
Removing all of the trim inside and out is a PITA, but it's the only way to be sure.
But if you don't want to pull the exterior apart, at least consider the interior, esp. if you're going to spray inhibitor in the panels. That's only one sunny day's work and you'll feel much better having done it, and if you have a mirror on a stalk and a good flashlight, you can check out the quarters and other trouble spots pretty well from the inside.
I think the most important exterior pieces to pull on 2nd gens are the rocker covers and the rear bumper. Most other areas, you can see from the inside. These are relatively easy to remove.
What you don't want to find when you pull the trim off the rocker panel:
Below the bumper, this is classic crx rust--note that the only part of this visible with the bumper on was the small rust hole along the wheel well. But this doesn't look that bad, does it? Before I took the bumper off, it was just a couple of coin sized rust holes.....
Unfortunately, the rust starts behind the panel, and it's much more extensive than what it appears to be on the surface. Here's what the back side of this area looked like after cutting:
But there's hope, after more cutting, sanding and wire brushing, it's ready for new metal:
And here's where rust started at a weld for a quarter panel replacement:
This is what the back of the rocker looked like, note that there's heavy rust along the weld:
And that rust ran almost two feet along the bottom of the rocker, here's what was left after cutting almost all of it all out (I still had some to cut towards the nose):
Aside from stripping the car completely and getting it dipped, there are two main approaches to getting rid of rust--brute force and chemical warfare.
First, get out your eye protection.
Obviously, you need to remove as much of the metal that's been affected as you can. In the case of body panels, the best thing to do is cut out affected areas with a grinder or similar tool--the metal's just too thin to do much else. Keep in mind that rust that has caused bubbling of the paint on the surface of the car is generally 2-3 inches wider in all directions on the back side of the panel.
Sanding works well for surface rust. For the rougher sections, 80 grit works well. If you're trying to remove surface rust, 180 or 220 is better. Keep in mind that when you're sanding you also removing good metal, so whatever you're sanding is getting thinner. For tight areas, you can get a mini sanding disc that fits drills, the pad is about 2" in diameter and screws into a hard plastic base.
Exposed rust can also be wire brushed away. If you're wire brushing, get a good one, cheap ones throw wire after a while. Brass brushes work well. Wire brushing is a bit slow, but it does not remove good metal.
In many cases as you grind off the rust you'll find a black metal underneath. I believe this is a variety of ferrous oxide, resulting from exposure to water and limited oxygen. This metal is still pretty strong, but is porous and will, if exposed to the air, quickly turn to ferric oxide, so you should cover these areas with a good anti-rust primer.
Most anti-rust compounds rely on phosphoric acid or similar compounds that convert rust to a more stable compound, such as iron tannate or iron phosphate. After conversion, the remaining oxide is hard and chemically stable, and can be painted or otherwise treated. Most of these compounds are pretty strong stuff, so use gloves and protect your eyes.
Rust Remover (Naval Jelly)
Usually a viscous liquid, you brush this onto the rusted areas and let it sit for 1/2 to several hours, then scrub it off and rinse thoroughly with water. I've had some success with this, but mechanical means are better.
Eastwood sells a rust dissolving liquid. I really like this stuff for certain applications. It's pretty effective at removing rust from nuts and bolts, just submerge the parts in the liquid overnight, and most if not all of the rust is gone. Very good if you have small pieces that are rusted together.
Also, NozHayr pointed out in another thread that coca cola can remove rust, which makes sense because it contains a small amount of phosphoric acid. And it's cheap.
A milder treatment, usually used as a primer. Very effective on lightly rusted parts, these bond to and convert rust, leaving a paintable layer.
These are paints that are formulated to contain rust, so that the metal below is not exposed to water or air. Basically their purpose it to stop the advance of rust. Example include:
Eastwood's rust encapsulator
, a lacquer based paint that I believe contains a mild rust convertor and glass micro beads. This comes in a variety of colors, and I've used it with good results. It's difficult to sand, but easy to work with, and can be removed later if needed with lacquer thinner.
, this comes highly recommended by the folks on the autobody shop forum.
, highly recommended by members of the community. This forms a porcelain like finish that is sandable. I haven't tried it, but from what I've read it can be tricky to work with--it can warp thin metal if applied too thickly and cures with moisture (so it can set up in the can after opening). But it's said to be very good stuff at stopping rust.
Rubberized paint that is suitable for the underside of the car, and inside of wheel wells.
Waxy substances such as Waxoyl
or Eastwood's rust inhibitor (they sell a kit with this compound and undercoating, and a gun with 2' wands), these compound can be poured or spray inside of body cavities to coat and protect metal. These are much better than the PWT method of pouring in used motor oil. If you're on a budget, you can thin these down and use a pressurized garden sprayer with a wand to get into body cavities. Make sure you don't stop up weep holes such as those along the bottom of the doors and rockers.
Some homemade waxoyl recipes:
Here's some rust that developed under the door and hood hinges:
And after sanding, there was a black metal in the areas. Rather than trying to cut it out, I elected to buy time with rust encapsulator:
Obviously, paint's also a rust inhibitor, but paints vary in terms of their ability to prevent rust.
Weld Through Primer contains zinc. The way it works, when you weld, the zinc coats and bonds to the metal and provides some protection against rust, much like galvanizing. It's highly recommended wherever you weld, but if you can get to the back side later, coat it with a rust inhibitor as additional protection.
Acid Etch primers are really not a paint at all, but rather an acid compound that converts the surface of metal to a phosphate compound resistant to rust. An alternative highly recommended by the autobody shop guys is Picklex
, this can be used on bare metal after sanding. There are also acid washes that are milder and perform a similar function, but are not as effective. Consider what you're going to be painting with, however, as some primers such as slick sand are not recommended over an acid base.
Primer is not recommended without a top coat for long, primers are porous and allow water and air through over time.
Rattlecan paints are generally applied thinly and provide minimal rust protection, so if you're using this, lay down several coats. Working outside, I used rattle can primer to spot protect areas since I was working outside, and with less than three coats, rust would come through the paint after a heavy rain.
Rustoleum and Trimclad are specially formulated for rust inhibition, so they provide a good level of protection, for more information see the various $50.00 paint job pages:
Oh, and if your rex still has that rubber trim covering the wheel well seam, take it off and hose that area out....