I have a 14 year old window that has some type of substance that I cannot remove. It is black and looks like someone took a 3/8' brush and accidently ran it across the glass. My first thought was paint or glue.

I have tried a razor scraper. I get fine dust but no discernable improvement.

I have tried acetone and have tried paint thinner. The rag got some black on it but the substance is still there. I tried scraping after each treatment, hoping that the solvent would soften it.

At this point, it is very smooth, barely raised from the glass but very noticeable.

Any suggestions besides using a hammer?