If you laid up the glass with epoxy resin,you'll need to use an epoxy fairing compound.
For the most part,polyester based resins and putties will not cure properly over epoxies.
The System three Quick Fair,is a beautiful fairing compound,and easy to work with.
Where are you located?
If you can't find it locally,you can get it from
www.mertons.com he ships everywhere and very quickly.
If you click onto where it says mertons catalog,you'll find it.The prices are not updated,on the online catalog,so call them to order.