The brushed finish provides a great base for most spray paints - I've never had a problem applying Hammerite type finishes with respect to the plate itself.
You mentioned multiple coats, so I wanted to caution about the recoat time issue that I have experienced. Many spray paints can be reapplied within a relatively short time with good results, but be careful about recoating after more than a short time. For example, I"ve had good results painting top panels in warm conditions recoating in about 15 minutes. However, when I recoated after a few days, even in warm conditions, the recoats tended to fisheye badly. I researched a bit and found this is a common problem - when the recoat times are given as 'within 15 minutes or after 10 days' - believe them. Had I followed label instructions more closely I probably wouldn't have had this problem.
Different paints may vary, of course, but this has been the sole source of top plate painting issues I've had with numerous BH kits.