I used normal multifinish wall plaster, left over from repairing a kitchen wall.
Changed to a bag of 'bonding' later, as it bulks up better.
The plaster is mixed with vermiculite granules, at a ratio of 30/70 plaster/vermiculite.
The vermiculite is only there to make it lighter, otherwise just plaster would weigh in at about 2 ton!!!
Cold weather, mix with hot water to speed-up the setting.
Place on with a normal trowel, finish off with a surform file and run over it with some 60 grit.
There really is no point it making it any smoother than that. Don't forget, you are finishing the outside of the f'glass NOT the plaster!
Slap on some latex paint & PVA, I mixed about 20% PVA into the paint.
I didn't wait for the plaster to 'fully' dry, as long as its 'crusty' on the outside, go for it!
Cover in f'glass, I used 3 or 4 layers, more in some places.
I loaded up with layers, giving me plenty of 'meat' to sand back on.