Yes, adding fiber glass to any resin based mixture will make the mixture tuffer. This goes not only for rondo, but also for most Smoothcast Plastics as well. The con of this method is that it would not be as hard as if you did a regular resined fiberglass coat, and then poured rondo on top. In my personal opinion not only is this fiber-glassing into rondo a waste of time, but not necessary at all. If you want to do a light coat of rondo, then add fiberglass with regular resin, then another coat of rondo. That would be much stronger, and cost you less money in the long run.