Firstly before preparing you need to identify if the fuel tank has ever had a tank seal fitted in the past, this will need to be removed before you apply the sealer, there are a number of ways to identify this.
1. The most extreme cases will be the fuel tank sealer starting to bubble as per the image
2. You may start to find small flakes of a varnish type material in the Carb Bowl, theses will also block the jets in the carb, so you may notice issues with how smoothly the engine runs.
Its not possible to use a "Fuel Tank Seal Remover" on Fibre Glass, as the chemicals are to aggressive and will damage the Fibreglass, Plastic or Carbon Fibre.
The only way to remove this is manually, by scraping the inside of the tank, one you have all the loose particles scraped of, the fuel tank can then be washed out with a degreaser, we always suggest using a diluted dish washer tablet, as these work very well.
After it has dried out, you can apply the sealer, its OK to seal over the old sealer, as long as it is stable, most POR15 and Kreem tank sealers fail, and we know that our product will cover these without any problems