Hammer-tone paint can take up to 2 months to cure properly.The reason for this is hammer-tone used to be known as stonegaurd and was originally sprayed onto sill panels on cars to stop the stone chips.Actually if one was to follow it right back it was made to use on the space shuttle,but we don't need to go that far back.
I'm not sure why you want to put clear over it,maybe you should have got a high gloss hammer-tone,although I don't know if you can get it in cans.
If it is because you like the colour.I'd just rub it with 400 grit wet and dry sand paper (Using soapy water with it)until you get a flat dull surface and then cover it with clear.Just make sure you use the type of paint,like enamel. I'm assuming the hammer-tone is enamel based so you would have to use an enamel based clear.That would work and should be fine for getting fuel on it.
