I found 11 of these on the roof of my Euro C180 1994 and what is called filiform corrosion which is veins spreading out under the paint coat from the rust centre. I have dug right down to bare metal and removed anything which looked like rust. Then treated with 2 types of rust killers/busters, dried the patches and used thick car paint of the same colour to fill up the surface dents. I overfilled the dents to ensure a good seal. When dried after a few days, I sanded down to the surface level and sealed with wax, polish etc. Unfortunately, the sanding down with 1000 grade left the paint shade different from the original if seen from different angles. Looked better if it wasn't sanded down. Think this will last me a few years before I manage to do a full respray provided I still have the car.