After dropping the fuel tank on our 1996 Buick Roadmaster, we found some oxidized ugliness despite the vehicle's near-impeccable maintenance record. Fortunately, the damage was caught before significant amounts of sheetmetal were lost, and modern technology has provided us with the tools to keep the situation from getting worse. A spray-on rubbery undercoating had been applied at some point in this vehicle's past, but as can be observed in the below pictures, it didn't provide adequate coverage. Moisture, salt, and dirt managed to get where the undercoated wasn't present, and the result was a lot of rust.
The situation looked bad enough as soon as we dropped the fuel tank, and removing a few of the heat shields showed even more corrosion. Left alone for another year or two, and this would have resulted in rusted-through sheetmetal. Fortunately, though, the damage is mostly cosmetic at this point, and can be treated relatively easily.
First, we'll remove the loose rust, paint, and undercoating, and then we're going to apply a coat of do-it-yourself truck bedliner (we got the idea from all the off-road guys that are using the stuff to cover their vehicles inside and out). Herculiner was on sale at a local home improvement store, so we picked up a gallon of it along with a few cheap paint brushes and a quart of zylene (this is the only solvent that's recommended for use with this product). Several other products are made specifically for this task from vendors such as Eastwood and POR-15. We've seen those used with success, but the DIY bedliner had the advantage of being available on a weekend.
To remove rust (and fingerprints), a 4" grinder with a quality twisted-wire brush makes quick work of the task. Before proceeding, it's wise to don a face shield and a dust mask, and a pair of tight-fitting mechanic's gloves is also a good idea (looser fabric gloves bring the risk of getting twisted up in the rotating brush, which isn't fun).
There's not much in the way of tricks or secrets here - it's just a matter of applying some elbow grease, and keeping body parts clear of the bristles. Watch out near edges, as the brush will tend to grab and jump somewhat unpredictably.
Heating up some of the loose undercoating in the wheelwells with a torch and scraping it off seemed like a great idea, until the burning material started landing on our arms and legs. A few singed hairs later, we decided to abandon this course of action. It's a dumb idea for a variety of reasons, so don't do this.
As it turned out, the brush did a wonderful job on the undercoating, instead of smearing and clogging up the brush like we'd though would happen.
This is why we suggested a face shield instead of some light-duty safety glasses.
There was also a portion of the lower rear quarter panel that had been "sandblasted" by debris thrown up from the rear tires, so we decided to tape it off and apply bedliner. It's low enough that it won't be easily visible with the car on the ground, and anyways, almost anything looks better than rust.
After taping off the area, we lightly scuffed it with a Scotchbrite abrasive pad.
After wirebrushing the frame and underbody (which, by the way, spectacularly fouled a previously clean garage), the area was wiped down with a damp sponge to remove any remaining dust. A quick hit with a powerwasher would have worked quite well, but we were hesitant to create a mudpit that we'd just have to lay in while completing the project.
Herculiner recommends using no solvent other than zylene when preparing a surface for use with its products, so we engaged in a rare moment of following instructions and did exactly that. Specifically, it's stated that the use of any alcohol will prevent the proper curing of Herculiner, so definitely avoid that.
Clean, oil-free rags are also a must, and Scott's heavy-duty disposable rags work well for this sort of work.
Wearing thick neoprene gloves is probably a good idea, as zylene strikes us as something that probably shouldn't be allowed to be absorbed through the skin. Good ventilation or a respirator is also a requirement.
Stir the material thoroughly, making sure to mix in all the rubber particles that are lying in the bottom of the can. Herculiner states that its bedliner can be thinned with up to 10% zylene if necessary, but we applied it at full strength (presumably, thinning would be highly recommended if one attempted to shoot this through a spray gun).
Starting off with a small roller, start applying the material to the larger open areas.
If the roller starts sliding over smoother areas, just lay down a thin skim coat and come back with a second application in 15-20 minutes. Interestingly enough, the thick bedliner will go down easier on the areas that have been roughed-up by corrosion.
Tighter spots will require the use of a brush, which doesn't do a great job of smoothly applying the thick bedliner. Try to restrict its use to corners, and use a dabbing motion.
Removing uncured bedliner (and other similar polyurethane-based coatings) from the skin will require some rough scrubbing with a harsh solvent such as zylene or acetone. Removing the stuff after it dries is damn near impossible. It'll soak through thin cotton (such as t-shirts), and thinner rubber gloves will eventually be ripped open on all the sharp corners of the underbody. Take this into consideration when dressing for the job, and don't say that we didn't warn you.
A bit of Dupli-Color's spray-in bedliner was used to reach a few select areas that weren't easily accessed via brush or roller. This didn't seem to go on as thick as the roll-in bedliner, so we tried to limit its use.
When finished, make sure to thoroughly clean the rim of the bedliner can before replacing the lid. If this isn't done, it'll be damn tough to remove the lid upon the next use. Mop up any spills immediately with zylene, because the bedliner is near impossible to remove from most surfaces once it cures.
During this session, we concentrated on the rear portion of the car, and covered the area that's normally inaccessible with the rear axle and fuel tank installed under the car. The next round of work will take place when we drop the exhaust for replacement, since it's currently obscuring a few areas that we'd like to cover. We're quite happy with the way that the quarter panels turned out (it looks like crap in the above photo, but that's because the car was filthy after all the wirebrushing), and will likely apply the same treatment to the rocker panels before the paint in that area takes on too much damage. We also see ourselves applying this to the underbody of the rest of our fleet as time permits, because the material and labor cost of this project is far less than that of extensive rust repair in the future.