If you're going to all the trouble of bare metal you gotta do it properly. I just posted a load of tips for a guy doing a s2 underbody.
have a look at that it listed in the forum "listed by model"
However if you do it in a overpaintable stonechip ie not underseal you can paint it whatever colour you like.
just my opinion but i wouldnt brushpaint it in hammerite.