If you've read the threads here you'd already have the answers to your questions, because this comes up every month or so.
This advice is based on what I’ve learned from OEM corrosion experts over the years: The whole point of rust protection is layers. The lower layers are there to help the outer layers to stick to the surface. The outer layers are there to protect the lower layers from the elements. All of the layers will wear off over time, so the more there are (and the thicker they are, and the better the bond between the layers) the longer that time will be.
Toyota bodies do very well against rust. However the truck frame doesn’t, because the factory coating suffers from lack of protection at the welds, and at the sharp edges of the steel frame. Rust starts at those places first.
Note that the #1 protection against rust is first getting the metal perfectly clean, then protecting it against re-rusting.
If you apply Fluid Film to a NEW vehicle, before it is exposed or begins to rust, and keep it touched up each year, that alone will do an excellent job of protecting for many, many years. However, if you try to only apply it over existing rust it will not be able to protect anywhere near as well (because iron oxide expands the steel, creating greatly more surface area exposed to the elements to continue rusting). The below steps to clean and re-protect first will go a long way to making up for that.
Steps:
1) Flush out frame rails with liberal amounts of high pressure, hot, soapy water (jack up one end and spray through all of the holes until the water runs completely clear, then repeat going the other way). Get all of the dirt and mud out of there so you can treat/coat the steel.
2) Remove as much red rust as possible (wire brush/sand/needle scale/media blast the parts in place, or better yet, remove as many parts as possible and sand blast and re-coat separately before re-installation).
3) Apply phosphate wash (Sherwin Williams sells Ospho by the gallon, ACE hardware sells it by the quart), spray it on with a cheap, disposable garden sprayer, or a hand pump sprayer. Flood inside the frame rails and chassis parts to make sure all inside surfaces are coated.
The phosphate wash cannot “save” heavily rusted metal, but it will convert any remaining, microscopic iron-oxide after removing as much of the rust as possible, into a more inert iron-phosphate. But note that is still reactive and will not last without some kind of covering protection against oxygen and water.
Phosphate will attack zinc plating, so keep away from any fasteners which are silver colored.
If you use a metal spray gun, Ospho will tend to etch away the steel inside its nozzle, so that is not recommended, for the good of your spray gun.
The Ospho wouldn't react with painted steel, plastics or rubbers, so I did very minimal masking. Clean up was a simple wash, afterwards. Use a tarp to avoid leaving white spots on the driveway. It is relatively safe for the environment (its what gives Coke is sting), if you get it on your skin will feel like a sunburn.
4) Paint cannot stick to oily/dirty surfaces, so clean, clean, clean with hot soapy water, rubbing alcohol, brake cleaner, etc. until the rag comes back clean. If you have oils like Fluid Film already on the frame, clean that off with oven cleaner until the surface is ready to accept paint.
5) Paint all surfaces with a good quality chassis paint. Note that most chassis paint is not UV stable, so an additional coat of top coat paint on top of it can be beneficial. Normally, UV protection of the areas not often exposed to the sun is skipped by OEMs, but we can add a UV protecting top coat to help prevent degradation of the chassis coat from reflected / indirect UV over time. Also note that applying paint over rust will not work, that will actually accelerate the rusting as it will hold salt water against the rust.
6) After the top coat of paint is fully dried, protect the paint with body wax. Products like Amsoil Metal Protectant, Cosmoline or 3M Cavity Wax will dry to a semi-hard, non-tacky surface. The 3M Cavity Wax comes with long straws with a 360deg spray nozzle to treat inside of frame rails, suspension arms, etc.
Note that if applying cavity wax by aerosol spray, use 3 heavy coats to equal a brush on liquid wax coat.
Note that cavity wax is petroleum based, so keep it off of rubber parts which would be damaged by petroleum distillates: suspension bushings, wiring, parking brake cables, tires.
7) Cavity wax will dry to the touch, but can crack over time, and is not self healing. So, once it is dry, go over it with a lightweight liquid rust protectant like Fluid Film, BoeShield, or Woolwax. These are lanolin based, so are OK to get on rubber, plastic, etc. but don’t get on brakes (obviously), or tires (slippery). FF is great stuff, but it does tend to rinse off, so should be touched up each fall. I use a cheap underbody coating spray gun to apply FF in bulk and it works great, and is easy to control. Aerosol cans are more expensive but are handy for annual touch up.
8) Any time anything like this is done, go back and make SURE all of the drain holes are still clear (not clogged with undercoating) to prevent pooled water causing corrosion after all.
Additional Thoughts:
- When lanolin based coating goes on, it smells like a sheep (it is lanolin based, so it is not harmful to rubber, plastics, paints, or your skin), and it, at first, looks a little like old mayonnaise. But, after a day it will turn clear and the whole underside will look black, and be pretty well protected.
- After Fluid Film application, it will drip (especially if you made sure to flood the frame rails through all of the available openings to coat all surfaces inside). This will make oily spots on your driveway. A fair trade, compared to rust. Over time the weather will wash it off the driveway.
- In the future, if you remove interior trim for whatever reason be sure to also coat inside those body panels too, for additional protection of those areas.
- Note that FF can be applied with a bulk sprayer, from a gallon can, the first time, then after that first big application each year a single rattle can and some long straws, and about 1 hour of time is enough to re-do the whole underside. For it I don’t even bother to mask or do any cleanup, simply sweep of a comfortable part of the driveway, slide around on a piece of cardboard, wear dirty clothes and then let the weather clean off any overspray/drips from the driveway when done.
- Whenever any fastener is removed, you can use some anti-seize on the threads before re-installation (note, when using it, which is a lubricant, reduce torque by about 40%, keep a record and re-check at that new torque value after a few 1000 miles just to be sure).
- Woolwax is a bit more expensive than Fluid Film but supposed to have no smell. The FF smell goes away after a few days. Boeing developed yet another version, also based on sheep lanolin, called BoeShield, for aircraft corrosion protection.
