You can lacquer over Hammerite for extra gloss and durability but you must use an enamel lacquer, Halfords sell it, or it will react!
Also always best to leave the paint for at least 48hrs before applying the lacquer, always paint a small test panel to see if it will still react.
I usually paint, leave at least 24hrs then apply decals, leave at least another 24hrs to cure then 3x coats of lacquer