I've had issues storing my restored MK2 jag in the same garage as the one i'm restoring, I always put it outside when grinding, welding etc. but about 12 months ago I noticed that little specs of metal had found there way onto the paintwork and were starting to rust.
Fortunatly as these were not hot as the car was not in the garage when the grinding or welding was done they were just on the surface but removing them was a challenge as you don't want to be trying to polish the car or compound it when its covered in little bits of metal (unless you like the swirl effect!).
I spent a day on it to get rid of it and started parking the car much further away from the garage rather than just outside the door and everything was fine.
When we had our house rephotoed to sell it (again with the previous sale having fallen through) the estate agent got me to move the car out of one of the photos and I left it outside the garage where it sat for a couple of days. I had forgotten about the previous problems but when I looked at it again it was once again covered in tiny rusty marks.
Rather than go through the pain of removing them by hand I looked at other options and one I found was to use this:
I found a thread on this an Bilt Hamber had replied and confirmed that it would be safe on paintwork so I ordered some, I tried it on a small section first and it worked really well and the paintwork was undamaged so I did the whole car, sprayed it all over, wait for it to go red and then hose off. Problem seems to be solved in less than 10 minutes.
I think you need to be careful and try it on a small area of paint like I did to make sure it doesn't mark it but in my limited experience it has been a real timesaver.
This is what it looks like mid process: