Like anything there are a few ways to do it, depending how thick you want it, I'd either:
Use chopped strand mat, but only one or two heavy gauge layers, then go for progressively finer and finer weight mat to keep as smooth and tight a surface as possible, then finish with a layer or two of surface tissue. You'll still need to sand and fill, but with care and carefull attention to the overlaps you at least reduce it. A good roller will help working the resin in and assist in not overloading it.
Alternatively - trickier, but get it right and it well get you a much better surface/thinner but still strong panel, is to use woven roving type may instead of chopped strand.
A lot of (amateur) aircraft build is done this way, inside out compared to kit cars, as a gel coat onto mould + backing CSM mat isn't always used, instead, they do exactly what you intend and build the covering up from the bottom, ending up with a twill mat surface that can be sanded and painted.