I had a job on a newly restored victorian house, where the bricks were more like sponges! As Andy said the steam cleared the majority of the paint. The darker the paint the more likely it is to require a chemical to remove the 'ghosting'. in this case it required a second application of Ghost ( good stuff!) but the owners were over the moon with the result, especially as I cleaned it all without damaging the bricks with a high pressure jet!
On the pricing side the customer was very gratefull that their house was in good order and so paid perhaps 3 days money for 11/2 days work!...A survey, I found was always required, to find out what surface the graffiti is on