A quicky, just remember not to mix your resins.
smooth on is a Polyurethane resin (I like to call it polyU) and the resin you use for fiberglassing is polyester (polyE) resin.
They have two separate mixing methods (one by volume, on by weight) and different cure times (PolyU cures a lot quicker than PolyE, which is why a lot of helmet makers use it, good results quickly = more helmets ready to sell.)
However, I personally would say that PolyU is best for mold casting and mold casting only, it's not a reinforcement material and, whilst it's plastic based,(so it can bounce off the floor) it shows its strength when it is thick, hence why the M-DSB helmets were chunky in the cross section. So layering on a thin portion of PolyU i going to lead to cracks or worn out amour.
PolyE is quite surprising in that in thin layers it can be quite flexible, but really takes on diversity when you add filler powders to the mix.
To save me from boring you all to death with my chemical experiments, if you want to fiberglass, buy a fiberglassing kit atfirst until you get used to how the material reacts. Remember fiberglassing is adding the strength and rigidity and should not be your outer coat. That's the gel coat, which can be as simple as, PolyE resin with some talc mixed in.
Hope that helps you.