Sulfur Doped Titanium Dioxide
Regarding why your ceramic material did not get coated nicely on the fiberglass, I think there's a problem of adherence between the fibers and the the ceramic. You need to prepare the surface of the fibers by etching for example before coating. They are often shielded.
Regarding why your  sol-gel can't be calcinated at $500°\mathrm{C}$. You need to figure out the proper temperature of calcination by monitoring the phase appearance of the ceramic materials by XRD diffractometry. You start with the XRD diffractogram of your as-prepared sample(normally, it should be amorphous). Then, you treat your sample at different  temperatures within different periods of times (you have two parameters to find: Temperature and Time). Unless you are working on a published article that has addressed all these issues.