When you are looking to paint over outdoor graffiti you will need to choose a product that both primes and seals the wall that you need to paint over. This will ensure that any stains from underneath the paint do not start to resurface in the future.
Before the surface is painted, you will need to make sure that it is dry. Secondly, make sure it is free from any other contaminants. Brush off any loose material. Any mould or plants that are growing on the wall should be removed using a mould remover. Any cracks should be filled and then sanded down so that they are smooth. When all of these steps have been completed, you will need to make sure that the moisture content of the surface is no greater than 18%.
When the surface has been prepared, you will need to apply a coat of primer. This should be left for at least two hours before the exterior paint is added.
You may need at least two coats of the exterior paint to completely cover the graffiti. Depending on the brand of the paint, you may need to leave at least an hour between coats but you should check the manufacturers instructions carefully in case more time is needed.