First: use alcohol or gasoline can be, but slow, expensive.
Second: With a blade is OK, but a square glass to engage in most of the day.
Third: use the hair dryer with both slower.
Fastest and cheapest way,
Use "Shuangfei powder" to rub the glass once, then use "acetone water" to clean, just fine.
Acetone water 7 yuan 1KG, a square of 500 grams is enough.