The weight gain is minimal, each coat of paint is as thin as a sheet of paper when dried. I would be inclined to sand, which gives the benifit of smoothing the lines and removing minor ripples. The original fiberglass is not "glass smooth"; sanding gives you the opportunity to create the glossy smoothness which enhances any paint job you put on your car.
Storm titanium 2005 Elise with carbon fiber spoiler, BWR sway bar, BOE toe links, gPan2, LidBone, sector 111 wheel studs, Grey RAC Monolite wheels with toyo R1R, Team Dynamics 1.4 with Hoosier A6/R6s, Katana2 Supercharger, ECU with 265 flash. PPE headers w/decat, CEL angle eliminator, Silent touch catback, Rear Panel Eliminator, HKS intake, V2 fuel tank, Walbro255 fuel pump. HID lights.