its probably difficult to get off due to vapor leakage
I find that if the screw cap was not competently tight, it will send out moisture at cap thus seizing the cap to the inside of the flitting that cradles the cap ( located on the inside of the cone)'
Dents are difficult to get out unless your an experienced metal worker, it can costs almost as expensive as a replacement lamp.
Painting or chroming is an option
For chrome, they melt and apply led filler, sand , polish, copper plate, sand, polish again,then chrome
when i got my lunar new from mathmos back in the late 90s the cone top was glued with a rubbery stretchy glue, i had to get a very thin blade under the cone lip and slide it up and around the bottle to split the glue
Early Lunars were glued inside. Later ones were not. Make sure that you remove the bullet tip first before trying to remove the cap.
id forgot the screwed bullet tip on top of the cone been years since i had the lamp, remember unscrewing it and the screw was stuck on the bottle cap