The boat was 20 years old, 2500 hours on the engine. Spent all the time in the Pacific NorthWest. I usually add BioBor before winter and generally (but not always) have the tank full going into the slow season. Here is a photo of what I found after pumping out the fuel through the normal intake. From this point on I used a pick-up probe pretty much as described by Dan y except used a brass barreled hand pump to get out the last gallon or so. (At least I think that is what it was since I have never had liposuction ;o) ) The tank walls were clean. The dark patches at the bottom were a mixture of some soft, flaky material floating loose that was slurped up by the hand pump and some hardened deposits on the aluminum. I tied a rag on a larger dowel and made a mop to scrub up the bottom that picked up most of the rest. I was quite surprised it was as clean as it was but like others I had worries in the back of my mind.