Is that to give it a better finish or just to give extra protection.
My powder coated Raleigh doesnt look brilliant, a little flat perhaps..but despite circa 4 years of all winter commuting, the coat is as good as the day it was applied.
If its purely extra protection, its not neccessary IMO. That said, if its applied by aerosol, it wouldnt cost that much anyway as a DIY thing.
Mine was done by a local Industrail company specialising in metal protection, zinc plating, powder coating etc nothing special, just a 'normal' coating. I'd never considered its thickness, but its withstood every knock and bang and several offs without a mark.
You would need to prep the frame prior to laquering it, and as I suggested earlier single pack is not very durable. You would certainly not make a powder coating any more durable by putting anything on it.
Also on a bike frame you do not want the powder coating too thick, as it will split and craze when the framed flexes