Where plaster / render / drip beads are used to create crisp and square edges on buildings you will often find that they go rusty and discolour paintwork.
The reason behind this is that the cheapest beads are galvanised and when they are placed in a damp and alkaline environment any damage to the zinc covering will allow rust to start. Once the bead starts to rust there is little that can be done apart from some cosmetic work, or removing the bead and starting again.
The best way of stopping rust from affecting your property is to use non-rusting beading in the first place. You can use stainless steel or plastic beading, or if you are doing plastering inside you can use wooden beads.