Use wd40 or similar, a flat block and a 1200 grit paper. Take the scratches away that youva made with your rough paper. Then clean off with brake cleaner and use an engineers flat edge to make sure you haven't fooked the block and it's still completly flat, also while your at it Did you find the cause of your old head failing, and it's not necessarily your head gasket, did you look for a water leak to cause the over heating, or wasit the cambelt let go? Genraly if you haven't got the Knowhow to maintain or know what to clean your block with you shouldn't be doing such a job, next time get it done properly, in my experiences. This type of job more often than not ends up in the garage anyway to be finished and also often costs more than the original job!
I'm all up for diy, but have a knowledge. Alot will say only one way to learn, and I agree. But if this gives up in 3 weeks time and your at the side of a motorway. Your safety is much more important that the few quid you MIGHT have saved, atleast have somone who does
Know what they're doing helping you.