Rondo is good, but brittle. I've used it before and recommend using it for hard-to-reach places where you can't get fiberglass cloth into, such as the brim of a helmet or etc...
I would also recommend using it to smooth the inside of the helmet a little bit over the top of the fiberglass. That will prevent you getting poked by loving sharp fiberglass splinters of death. You don't have to do that of course, but it works as a sealant against fiberglass shards.
If you're going to go the rondo route, I recommend following the mixture of hardener to product for both resin and bondo, and going in small batches. If you make too much you run the risk of gloppy drips and messy patches that are totally avoidable. And just like the fiberglass itself, put in more than one layer.