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So in the absense of actual information about the effect of a light film of WD-40 on the brake rotors - aside from a dramatic warning from Shawn D, a newspaper article suggeting it's OK, the same tip on the WD-40 web site, and numerous references to people using it when putting cars up for storage - I tried it.
Here's my experience, on a car with clean/smooth rotors, with only the outboard surface of each front rotor sprayed:
- No perceptible change in braking behaviour, including the first stop (on an empty road), from moderate speeds (70 km / 45 mph) using moderate pressure (not close to approaching lock up).
- The pedal felt neither grabby nor spongy. It did not feel like I was trying to stop on ice.
- The second time I braked, I deliberately tried to see if I could achieve lock up, again braking from 70 km/h / 45 mph. No problem achieving lock up. (Dry, clean asphalt.)
- The film did not "dissipate from the rotor with the first applications of the brake" - at least not from the speeds and the braking pressure I was using. It took a total of five moderate/hard stops in a row before the film was gone. (At that point the rotor was too hot to touch). Maybe if you verged on lockup from high speed, going downhill, to a full stop, it might work in one try.
Is it possible that controlled testing could reveal increased stopping distance, before the film has been heated off? Of course it could. Is it a dramatic difference? I suspect not. (Now feel free to debate the relativity of safety.)
Is it possible results would be different if both sides of the rotor were sprayed? Of course it is. I didn't try both sides.
After trying this, would I personally do this to my rotors if I knew I was going to leave my car parked outside for an extended time? I'd consider it. It's still a pain in the butt though, owing to the smooth wheel covers and accessing the inboard surfaces.
Would I advise anyone to do this? Of course not. It's clearly unsafe.
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