1.   Lighter fuel can be used to remove more stubborn stains.

2.   Use an abrasive pad for stubborn stains.

3.   This treatment removes the most stubborn stains.

4.   For stubborn stains you may have to apply the phosphoric acid full strength.

5.   In extreme cases stubborn stains can even be removed by using an abrasive type fiberglass cleaner or auto cleaner, or even a kitchen abrasive pad.

6.   Stubborn stains may have to be hosed and scrubbed.

7.   Stubborn stains may require a stronger cleaning agent.

8.   Stubborn stains may be removed with solvents like rubbing alcohol or mineral spirits.