I currently live in South Africa with a SA passport. My grandfarther was born in France and I wanted to know if I would be able to apply for a French passport through my grandad?

Maybe or maybe not... if your father never lived in France, citizenship will not be inherited to his children. Contact the nearest embassy or consult French immigration lawyer.

There is a 50 year rule that the French law has for "loss by disuse" - see details at:

Conditions for the obtention of the french nationality — EuskoSare (http://www.euskosare.org/euskal_herria/euskal_herrian_bizi/french_nationality)