Like anything it depends on the severity of the damage to the glass. For light marks you can use a heavier clay on the glass. You can use varying acids from vinegar to as strong as diluted wheel acids. Some compound the glass with a wool cutting pad and heavy compound. With the acids becareful of the acid run off damaging other areas such as paint, wheels and trim. Most of my customers tell me they offer glass work without guarantees of quality to the customer. They will do their best but it won't be perfect.