I bought some 100LL avgas from the local FBO—used their self-serve pump. I wanted exactly 1 gallon. However, someone had just fueled his high wing plane and this left some gas in the hose. So, I got 1 gallon plus—just not sure how much extra. I went back to the hangar and poured the gas into the right tank. I sloshed it around lifting the end of the wing up and down and did the same for the tail wheel stinger. I had marked a jar with .5, 1 and 2 quart levels. I set this under the sump drain and put a coffee filter in the opening. Opened the drain and filled to the 2 quart mark. I poured this back into the empty gas can and drained another 2 quarts. There is still some left in the tank, which I’ll drain tomorrow. The coffee filter only had a few small pieces of trash—appears to be some pro-seal residue. Apparently, the tank was pretty darn clean. I’ll repeat the same for the left side tomorrow, then test the output of the electric fuel pump. The hardest part of this entire session was holding the drain valve open while the fuel drains out.