Lizard skin ceramic coated my car, for the surface area covered, it's very light...Assuming you are only after thermal insulation. The ceramic coating is only 5.7lb per gallon and I probably used about 1.5gal from halfway up the firewall to the rear valance and the underside of my roof.

I plan on running clear ceramic tint this spring, which reduces the infrared solar load coming through the glass by 40%. I expect that to be a gamechanger in a classic car, a ton of heat comes in through the glass.