Why does rust need to be removed?

Question

I previously asked this question:

Is this rust or not, will it spread, how should I repair it?

People are saying one needs to sand down to bare metal then paint over to prevent rust from spreading.

However it seems to me that the day I had the collision that rust appeared instantaneously. This would mean it was already there under the paint work and didnt appear over time.

If so why cant we just paint it(as it would have been before the collision), why do we have to sand it down to bare metal.

I could be wrong but I recall that appeared as a result of piercing of paint and so that might be the original metal?

Answer 1

Exposed steel will rust very quickly - in a matter of hours in a damp environment. Just look at any car that's been parked up for a few hours and chances are you'll see spots of rust on the brake discs - these are cleared off the first time the brakes are used.

In the photo in your other question, there is only a very thin film of surface rust, which fits with it having only been there a short time. If it had been rusty for longer you'd see a deeper, darker colour and more pitting and texture to the surface. If you don't treat or remove the rust, this will continue to develop, leading to the new paint flaking off the surface and exposing the metal further, eventually leading to more significant corrosion.

As it's only surface rust, it will be very easy to take it back to bare metal - a wire brush in an electric drill will do it in seconds. A couple of coats of a suitable primer (usually branded something like "Anti-rust primer"), then two or three thin coats of body colour should sort it - for that area you'll only need one can of each, with plenty spare...