For normal fibreglass surface to be smooth, you need to smooth them up by yourself.
There are several ways of doing it, either you sand down the inperfection of the surface completely then apply bog or putty filler over the surface and sand again. The other way is to just fill the whole surface with bog or putty filler entirely and start working on the surface, how would you want it to be.
But of course you cannot use this pieces of fibreglass that is covered in bogs and puttys on your car. You can and there is no problems with that, but with the amount of bogs and putty you have applied to the surface, sooner or later, they will crack upon vibration or showing wrinkles on the paint surface in the future.
This is when moulds have absolute advantage to answer all these problems.
How do you make the moulds? Simple, with the already perfectly workout fibreglass panels (after the bogging and sanding to flat surface), apply thick layers of fibreglass over the surface and all around with some extension. When it is all ready, just knock away the old panels and you have a mould, from there you can perform as many duplication as possibles with a perfectly flat finishings.
But there is lifespan for the moulds as well. From time to time after many applications, the moulds can get out of shape, and the surface will damaged, soon, you will have to make and entirely new moulds again.
