I thought that also so when testing was complete I drained the tanks fully. Then I left the system open so most of what was left would evaporate. When it comes time to actually run fuel through the system I will waist a gallon or so by putting it in the tanks letting it run to the header tank and then draining the header tank until I see no contamination. As far as fuel testing I will do complete system leak testing with fuel before I mount the wings.