I use a Model Masters Lacquer over coat semigloss on the chrome before masking and then a lusterless (flat) of the same brand when all the main colors are done. This keeps your chrome nice and shiny but not too shiny. It also seems to help my paint stick better. The flat give that old dull look to the rest of the helmet. I am sure the brand is not important.
Anyone feel free to just in with and more detail as I am no expert here. 
You can get these in the can, thats what I think most of us here use!