Either Polish it with autosol every week or cover it with high temperature lacquer. Note the high temp lacquer doesn't cope on exhausts, but i imagine the block should be cool enough
Like these: http://www.ebay.co.uk/itm/2x-CLEAR-...925?pt=LH_DefaultDomain_3&hash=item51b9534845
You will need a few cans i imagine. Took a couple to do my exhaust, which has now all come off :