I was very dissapointed when my landrover rotted from the inside, as I had spent a lot of time painting the outside of the chassis (when I was supoosed to be revising for school exams). So from then on I have used waxoil, both black and clear. To apply it I found neither a hand pump sprayer or a compressed air oil gun would apply it well, even when hot. So I combined the two with a garden sprayer piped to the pick up pipe of an oil gun. (You need a tap though or the waxoil just keeps coming out). It gives a much better spray pattern of waxoil both inside and outside the chasis.
The transit stumped me though. I have the chassis cab and it had a metal strip spot welded into the bottom inside of the chassis rails. Rust had worked its way between the two parts, so the bottom of the rails had to be replaced. Waxoil had not pentrated the joint, even with yerly applications inside and out. With this in mind I turned to oil for the inacessible spot weld joints. Using an oil can, I have fended off rust from the doors and front wheel arches on a vehicle which was made in 1991.
Last week I tried a mixture of waxoil and engine oil. Its much thinner, which I hope will get into more of the hidden joints. It did rain oil on me while I was doing the job, but I was suprised how little finished up on the drive
I think the rule is;
If its rust free, waxoil will protect it really well and keep it that way.
If its light rust, waxoil will slow it down given yearly applications.
If its heavy rust, it needs to come off, be treated, painted an waxoiled.
If its internal chassis rust, then its oil, as its the only one that gets sucked into rust.
I wonder about using 'duck oil', or any of the really light 'non evaporating' oil? I have used old engine oil in the past, but its dirty stuff. Bearing in mind the cost of welding, why not squirt on new cheap engine oil or gear oil?