Ok, the fuel tank in my 1986 930 was replaced with a new Porsche tank the year prior to the previous owner parking the car for 9 years. Draining the gas and looking in the fuel level sender port showed significant sludge and rust buildup.
So it looks like the inside of the tank is covered in a nice thick brown velvet.......
I read through many Pelican horror stories of cutting tanks in half and sand blasting. I decided that I would first try to clean the tank with no cutting. I pulled the tank back in April, and have slowly been building a device to clean it.