Thank you for all of your help, but in the end I don't think I will use paint on customs - I remebered a video I had seen by the brothers brick here: http://www.brothers-brick.com/2013/06/30/50-shades-of-bley-iain-heaths-mad-lego-science/ where he uses dye to perpanently change the color of the lego bricks. You might find this interesting, as it would mean that you wouldn't have to worry about the brush slipping or anything like that with paint. You just have to have new-ish bricks, because scratches show up easily.