I am about to purchase a 22 year old (1990) Chevy C/K 2500 from a friend who is moving out of the country. I’m only going to pay $800 for the truck. I believe he has taken good care of the vehicle and it’s mechanically sound but rust prevention was never a priority. The fenders and tailgate are rusted through and some minor surface rust is present. I may or may not deal with the body rust as this is only going to be used as a work truck, hauling wood and trips to the gun range etc. My question is about Chassis rust. Underneath it looks to be a brown mess of rust as you would expect on a 22 year old truck. The suspension and springs looks good and the rear axle is much newer than the truck, but what is the best approach to protect this thing as best as possible without spending an arm or a leg? I know most people think undercoatings are a scam on new vehicles, but would it help on a 22year old chassis? Some say to spray old oil, some say DON’T do that. Some say to use a coating I can paint on, some say it’s a waste of money. I won’t be using this truck every day and I plan on doing as much work as I can myself, but I won’t be doing a body off restore or anything, but I don’t want it to just sit and dissolve either.
Try this: http://www.por15.com/
You really need to have a mechanic give the underbody and frame a thorough inspection. Rust damage creates dangerous vehicles, if the frame is badly weakened.
And in the condition you describe, there’s no stopping it now, I’m afraid.
I also would recomend POR15… Its great stuff, but I am going to tell you now, you are not going to have a good time putting it on.
If the rust is that bad I think you would be wasting a lot of time and money for little gain.
The explorer Robert Ballard has a plan to save the Titanic by painting it and I’m skeptical of that one too. 
$800 is a nice parting gift to your friend.
In my state this car is not inspectable(for great reason) and likely worth about $300-$500 in scrap.
Have it put up on a lift and take a relatively sharp object and peck at the frame. If it goes through this vehicle is unfit at the foundation and instant junk. If not don’t worry about it and drive it. I would not waste a penny on rust prevention on this vehicle.
The Farmer Method: Fill a cheap garden sprayer with drain oil and spray the entire undercarriage with it. Repeat every 6 months to a year…Warning. This makes a mess on the ground…
Even if it’s a Silverado with a 4-spd auto and 5L engine, it’s not worth $800. You shouldn’t pay over $400-$500 for it. You can lose the tail gate, but you might want to repalce the fenders. If the frame is unsafe, just walk away. You really can’t weld it to make it safe enough. It won’t be bad in just one spot.
BTW, do you have a state inspection to register it? I doubt it would pass. If you need one, pay for the inspection before buying the truck. It will likely require over $1000 to make it safe, and the truck isn’t worth more than $800 even if it is a loaded Silverado.
If the frame is solid now Caddyman’s solution may be the best option. That was a very common way to rust proof farm equipment years ago. I found it easier to apply the waste oil if it was diluted with kerosene. And although there has never been any need to salt the roads in my region I have seen cars from the north brought down that would sag when the floors began to crumble and pickup frames fold to put the bed into the cab. All manner of Rube Goldberg patches seem to fail to keep the scrap held together.
Paint everything that is rusty or use what is recommended in this thread. Do something rather than nothing to seal off the rust. Then: Do not drive the vehicle on salted roads. Park it out of the weather inside a well ventilated garage. Rusting will stop. Rain is ok but winter road salt is guaranteed eventual death to motor vehicles. I have a car older than your truck; has never been in winter salt and is not rusty. Another benefit of a non-rusted vehicle is that it is easy to repair due to lack of corrosion and worth repairing in that I don’t feel like I am repairing rubbish. If you live close to an ocean, the above suggestions will not stop rust.
Perhaps I over stated a bit the amount of Rust. The under carriage isn’t just Rust, I mean there is bunch but the steel is all solid, the only soft points are really the two rear fenders. I am going to get the truck up on a lift and check the frame really well. I understand I’m not buying something that is perfect, and I have no thoughts of turning this into any kind of show truck or a complete restore, I just want it to last so I can pick up wood/crap and drive down muddy trails to go shooting. My main question is about the undercoatings. Are they worth the time/Trouble/Money? POR looks good, but is worth it? Have you used it? Does it really work and last?
If the frame is bad, don’t bother with the title transfer, just tell your buddy to drive it to the local recycling plant and have them scrap it there and collect the money from them.
Pay close attention to the brake lines. If they’re rusty, replace them.
Rust is often like a cancer. The above suggestions are more effective if done before rust begins.
You can stop the rust anywhere you can apply oil or paint grease. If you are so inclined, the vehicle can last. The problem is, what is the present state ? IMO, it was not a good buy, as rust is the biggest enemy of an older car, not mechanics. The truck in need of a new motor is more valuable then one needing a new body. You would have been better off with something needing a new major componet but having a good body. Take it to a body shop and have it evaluated. Don’t expect much. As you discribes the vehicle, I don’t believe it is even inspectable in my state.
Undercoatings or paint will do NO good. Oil and grease only on cars already rusting is about all that works. Use a dedicate garden sprayer with new oil as waste oil will clog the spayer. Paint red grease on exposed areas on frame and attachment point areas with a foam brush.
Nothing that we say matters until you get it inspected by a mechanic. Let us know what they say.
The frame is the biggest concern on this truck. Be sure to check the framerails below the firewall. This is a common problem area on these trucks for rust perforation. If the frame is becoming Swiss cheese there or anywhere else, the truck is junk and worth about $500 to a recycler in my area (since it weighs a little over two tons), maybe a little more if it has aluminum rims.
After further inspection the frame does appear to be solid with only surface rust, so far. No soft spots or place where you can poke any holes. The Spring hangers are worn but still solid. The brake lines are newer and in decent shape, but are have some light rust in a few areas. The problems found are loose steering, Pittman and idler arms are worn, sagging driver door, missing Catalytic Converter and the shocks and muffler are shot.The truck is rough, there is no doubt about it, but what are you going to find for less than $1000 these days? I think I can get new shocks, muffler, Timing Chain Kit, Pittman and Idler Arm for about $500 and this would put the truck in decent driving shape, than it’s just a matter of stopping the rust as best as possible and cutting/welding some body steel to replace bad spots. Thoughts?
…I hope we realizes that the frame rust and sheet metal that allows fumes access to the cabin are equally important. There is a reason why sheet metal rust makes a car fail inspection as readily as frame rust. I would never buy a car with severe rust in the fenders and elsewhere, while I would buy one that needed a new motor. This discussion is upside down.
Look for a truck with a sound body that might need some mechanical attention. How safe is a car when you have to breath exhaust fumes or wonder if at any speed the body will collapse ? If you can"t afford more for a work truck, you will be throwing more away just keeping a rust bucket on the road while putting yourself at risk. Missing catalytic converter ? Your problems have just begun.
How many have made $800+ mistakes? I have made a few. But then there have been some really great treasures under the dirt and rust on a few beaters that have tempted me to turn loose of a few bill$.
I admit I’m not an Auto Pro, but I’m pretty sure no exhaust is going to find it’s way into the cab of a pickup truck from rusted rear fenders, unless the laws of physics are really different on redneck vehicles… The only rust in the cab area is a small hole on the driver floor board, easily fixed with a little patch. As for the Catalytic Converter, I know many people who don’t use them for performance reasons on street and off road vehicles, so I don’t see how this can be a safety concern, sure I may need to address it if I ever need to pass an inspection, but that is unlikely where I am. Again, I understand I’m not getting a mint condition gem, I get that, message received. If anyone has inside information on where to find these 20 year old trucks with mechanical problems and perfect bodies , please share the wealth, I’ve had my eyes open for months and hardly ever find anything worth a darn for less than $2500, so I am looking for something that will work, with a little elbow grease and $1000 or less. Still looking for the Grandpa truck kept under the shade tree in Texas for 15 years but it’s eluded me so far…