My tank got so bad on my 930 I had to clean it using electrolysis.
It took about a week because the battery charger that I have is only 10amps but it worked.
Do a web search for cleaning tanks using electrolysis and you should find all the info you need.
You can coat it after it is clean or just keep it full of fuel.
PS. I also added a screen filter to mine thatís up into the tank, the kind that is used on some models of sediment bowels.