even if i done it out of metal... same problem....
once the metal has been welded on and then the filler is placed over it.. when the doors are slammed shut.. CRACK!
soo theres no real escaping the cracks unless you have a flexible filler, and the metal would weigh more i think..
sooner or later il be a bit better with fiberglass so im planning to remake parts of the car out of fiber glass and replace them with carbon fibre parts.....