I recently received an email from a pilot who lives in a remote area where 100LL is not readily available.
He was thinking about buying a used tank that had been storing #2 diesel fuel. He was wondering if it could be safely used to store 100LL.
I debated the answer for quite a while as to whether it would be OK to use: Maybe, perhaps, or depends. I finally settled on depends.
First it depends on how clean the tank is. If the tank had been outside or the water not drawn off, there could be sediment and even bugs in the bottom of the tank. Therefore, the tank should be inspected and flushed out to ensure cleanliness.
There may also be some contamination from sediment and/or film on the walls. A good rule is that you lose at least one octane number per percent of diesel or Jet A contamination in 100LL, so a small amount of contamination should not be a problem.
If we assume the tank is clean, the next thing is how it is plumbed. The main problem with storing fuel is water. It can come from rain if the tank is outside. But the main source is condensation.
When a tank cools down in the evening, high moisture air is drawn in, and then during the night, the temperature can go below the dew point of the air in the tank. This will cause moisture to condense on the surface of the tank and form droplets that can sink to the bottom of the tank.
The tanks at FBOs are designed with a water draw at the very bottom of the tank, so that any water formed can be drawn off. FBO tanks also usually have floating suction that pulls fuel off the top and does not pull any water off the bottom or disturb any water that could be on the bottom.
It may not be practical to install a floating suction on a home tank. But you can install a water draw so that you remove the water before you refuel your aircraft.
Most cylinder type tanks have the supply port on one end about an inch or two above the bottom.
One solution is to install a tap on the very bottom and drain it or sump it before every refueling.
Putting a drain in an existing tank can be problematic, so another solution is to tilt the tank a few degrees so that the supply port is the lowest point. Then install an intake pipe in the top of the tank on the other end. Make sure that the intake pipe is several inches above the bottom so that you do not disturb and pull in any water sitting on the bottom.
Always sump the tank before fueling your aircraft.
Another point is do not pull fuel out of the tank right after having it filled. When a tank is filled, the incoming fuel will disturb any sediment or water that is on the bottom. So let it sit an hour or two before fueling your aircraft.
The system also depends on the filtration system. Some systems have a sediment bowl on the bottom so that you can see if there is water build up. This is a nice safety check for water.
On large commercial units, there is a pressure gauge before and after the filter. If there is a significant difference between the two readings, it is an indication that the filter needs changing.
On most private systems without gauges, you need to be aware of the fueling rate and change the filter whenever the rate slows significantly or on a calendar time basis.
Location is also important. Partial shade is good as it will reduce evaporative loss, as does white 
paint.
A final “depends.”
One of the aviation filter manufacturers developed a fuel filter that had water absorbent paper in it. This paper is the same absorbent material used in Depends diapers.
