I'm looking for something that doesn't need any surface preparation. I know in Ontario its common to spray the undersides of cars with used motor oil but I'm thinking there may be something else that can go directly on panels with surface rust.
Ask me! Ask me!
I'm in Ontario, and no! never! to motor oil, used, new, synthetic or otherwise. Too thick to wick into the nooks & crannies, and the used stuff has nasty stuff in it that eats metal, and is toxic.
I know exactly what to do.
From the really picky British car restorers, when POR-15 or it's competitors aren't good enough, two words:
Penatrol & Waxoyl.- Wash the dirt off the underside.
- Let dry. Thoroughly. Days required for all the nooks & crannies to be dry. Don't worry about stray dirt that finds its way on during the drying, it'll just get soaked with what is used to treat the metal.
- Penatrol oil paint conditioner. Brushed, rolled or sprayed on. Under $10 a quart in Lowe's in the States.
- Let the Penatrol dry. This takes a few days. Leave it a week to be sure. The solvent evaps off quickly, but you have let it "cure", as a major component (linseed oil) is a "drying oil" which means that it oxidizes to cure. It takes oxygen from the air, and from the surface rust it soaked into. Don't worry if you haven't got rust. It's a great metal prep/sealer coat.
- Once cured, you can top coat it with anything.
- But, you want to use the Swiss product of oil & wax emulsified together. Waxoyl. In rattle cans or larger cans.
Waxoyl
"Hard wax", for underbody undercoating
http://www.roversnorth.com/store/p-5727-waxoyl-hardwax-underbody-500ml-aerosol.aspxit's black, really sticky, takes weeks to fully dry, but it does skin down
"Soft wax", for inside doors, floor pan, but also the seams in the hood and around engine compartment (I use a bit on nuts & bolts so I can get them apart another year)
http://www.roversnorth.com/store/p-5726-waxoyl-rust-inhibitor-clear-500ml-aerosol.aspxthis never really dries
I know the second one is called "rust inhibitor" not "soft wax", but as the undercoating one is "hard wax" it seems that the other gets called "soft wax" by those in the trade.
You can use the Penatrol on clean metal or rusted metal. When
* you're doing a perfect restore job, or
* when you need to arrest rust development on something until you get back to it later, be that in five years (check once a year and give it a fresh coat if there's more rust).
Remove the scale first. And you can scrub it into the metal with a scrub brush for heavily rusted spots.
If it's a beater but you need it to last: brush the Penatrol on, leave a week, rattle-can the Waxoyl Hard Wax over it.
I brushed Penatrol onto the parts of my rebuild that I couldn't get to until later. It stopped the rust from progressing.
If I'd had Penatrol and Waxoyl I'd still be driving the '81 Eagle Wagon instead of it dieing of rust.