I was just thinking about rust on a brake rotor. I mean rust in the active area of the rotor where the pad would fall when the brakes are applied. I know that when the car is not being used and when there is moisture present the rotor will rust. There are many reports of rust forming in a short period of time, like overnight when there is moisture (rain for example) and the car is left outside. Now it seems that this light coating of rust that does form overnight might impede the formation of further rusting that will take place if the car were to sit for a second day unused in the presence of moisture (rain for example). If the car were to be used on that second day (rather than sitting unused) that light coating of rust might get removed by the application of the brakes. Using this logic is seems the rust which forms when a car sits for 1 month (or so) unused would actually be protecting the rotor in the sense that less of the rotor material is being consumed by the rust than would have taken place had the rust been removed daily by daily driving.
Now, when the car is driven after sitting for 1 month (or so) it seems there would be a thicker layer of rust on the rotors that would need to be worn off by the application of the brakes. Now, if the braking of the car seems as good to the driver with this thicker (1 month or so) layer of rust being worn off what harm would there be? When I say the braking of the car seems as good I mean the braking distance seems the same. I mean, lets say that the owner, after returning to the car after sitting for a while ( 1 month or so) takes off the wheel and lubes the slide pins to make sure they are not frozen, removes lubes and reinstalls the pads to make sure they are not frozen, c-clamps compresses the piston to make sure it is not frozen, test drives the car to verify the braking distance, test drives the car to verify there is not a dragging brake (by various means). That is to say the brakes seem to be working as well as they did before the car sat for a while (1 month or so), only its just that the pads are wearing away that rust layer for a while. Would there be a risk in that?
What would it say that the brakes seem to be working as well on a rusty surface rotor than as with a shiny surface rotor? Could it be true?
Been through a coupe instances. One work truck brakes could lock up overnight, enough gas to break them loose worked out fine. Had a ranger I was selling, not sure about the rust pattern, but felt thumping in the brakes for 50 miles or so, after enough miles and hard stops all was fine
If you are asking which brake configuration is better, pad on un-rusted rotor, vs pad on rusty (in the active area) rotor, it seems like the first would produce the quickest stop. But it might not be that noticeably different to the driver. One thing that can confuse, if one of the pads have worn down to the rivets, it can sound like the pads are ok and the rotor is just a little rusty but it is actually the pad’s rivets are exposed, and the pads are in immediate need of replacement.
Look at all the cars that sit in car dealers lots for months on end.
Normally, rust isn’t a problem on the rotors.
It’s quickly removed when the vehicle is put into service.
Unless the vehicle has really cheap brake rotors.
Tester
I wonder if the cars sitting for months at a time on a dealer lot have anything done to them to prevent the rotors from getting rusted? I wouldn’t think someone would want to do a test drive only to experience poor braking even just noisy braking.
No they don’t do anything because it is not near the problem you think it is .
good to hear this is not a problem
It seems you raise a good point. If the car that has sat for a while does make a noise it may not be such a good idea to assume the noise is due to the pad wearing off the rust because this would leave one open to the possibility that a pad is wearing all the way down. Better to pull the wheel and inspect.
Interesting to hear that in these cases things worked out fine
I experience rusted rotors on a regular basis. I’ve noticed a quicker grab on the first stop many times. Usually the rotors clean up really quickly. Sometimes if the weather is bad rotors can rust enough to cause permanent damage in a matter of a few weeks of no use. Rust is never your friend.
Rust on the rotors is not much of as problem in most of the country. It is a big problem in the Buffalo area because of our heavy snowfall and relatively warm winter temps. that keep us hovering near the freezing mark and going from the high twenties at night and the mid thirties during the day. Couple that with the absolute carpet bombing of alt and brine and you can expect short brak component life. It doean;t just kill the rotors, it eat up calipers, caliper slide, it even get under the stainless teel clips and binds them against the slide.
Any rust preventative they might put on the rotors would be more likely to cause brake failure than rust. Products like that (oils) are used on replacement rotors so that they aren’t rusty when you pull the out of the box. That’s why we we spray brake cleaner on the surface and wipe it off before installing the rotors. No one sees the rotors on new cars and the rust wears off quickly, so they don’t coat them.
I like your mention of rust forming under the stainless steel clips. it seems removal of the clip and cleaning out under it should also be a required step in making sure the brakes are working properly after a car has sat for a long while.
With the big open wheels on many cars these days, the rotors and calipers are very visible. Manufacturers would LOVE to have rotors that don’t rust as they tend to cause warranty issues with new car buyers of cars that sit on car lots for extended periods. They also tend to rot away on cars that get little use, especially in rust-belt states.
I have suffered this on many of my cars. Replacing rotors and pads with 1/2 the brake pad left because the edges of the rotors rusted away…even with top quality rotors.
BUT… bare grey cast iron is the best, low cost, material for brake rotors. Cast stainless steel does not work nearly as well. Aluminum is too soft and if overheated becomes like putty. Carbon rotors are STUPID expensive.
Porsche has a tungsten-carbide coated rotor that doesn’t rust and lasts a long time. An iron-rotor brake replacement is $2400, the ti-carbide rotor is $11,000 but the carbon rotor is $32,000. 
If you want rust free rotors… you gotta pay the BIG bucks!
Last winter we didn’t drive our Solara for 3 months while we were traveling out of state. One of the brakes locked up. I had to bang on the rim to get it to unlock and then I had the brakes inspected for rust. They were OK. We live in upstate NY where there is lots of salt and moisture…
Surely this must be evidence of a conspiracy!
Or, In other words, anything that someone doesn’t like, or doesn’t agree with, or simply can’t comprehend as a result of poor schooling, must surely be the result of a conspiracy.
I have not experienced light rust on the rotors significantly impacting rotor life. Pitting and bubbling yes.
The term “lot rot” denotes the degradation of cars that sit undriven on car lots. Rust and pitting of rotors are prime indicators of lot rot.
It can be a problem if you don’t drive much. I never had that problem living in Syracuse (or north of Syracuse) or my relatives still living there don’t have any problems. Any rust that forms even as long as a week ago will be gone within 5 seconds of the first time you hit the bakes over 30mph. It’s just surface rust that brake pads can easily wipe away.
Syracuse has colder winters than Buffalo and north of Syracuse has much colder winters. Salt is more active at warmer temps.
Th` Buffalo area has a bare roads policy, they keep salting until the pavement is bare.
North of Syracuse, they don’t even attempt that . Also , they use more sand in the mix or even all sand if it is cold enough.`They expect people to be able to drive on the snow and
ice in the north country.
I made my living for many years driving Watertown turns from Buffalo and to Montreal vis Watertown ,Chateaugay and Champlain.