Ok bud, first off, see what you are using the helmet for. If its only for display, and your not trooping in it, it may not actually need a clear coat. Mine is for display, and as I got a really nice finish throuh the airbrush anyway, I decided not to clear coat mine. But ultimately, that will come down to what you are using it for, or just preference..
If you do decide to coat it, stay away from gloss. Some see the lids as having a slight satin finish, so that can work, I personally prefer flat. Just not gloss. Out of a can, testors dullcote is good, but do be warned - it will dull your metallics. Which some like for a more weathered feel, but I personally prefer bright silver. Most flat coats will do this though. Brands like humbrol also provide clear coats you can spray through the airbrush - and will be needed to thin down a fraction with white spirit. Other than that, you can try cheaper spray can versions in bunnings, just make sure they are compatible with the type of paint used - enamels, acrylics or laquer. obviously, always do some test strips before applying it to your lid. Best of luck m'man! 
