On September 4, 2004, about 1630 eastern daylight time, a Beech 95-A55, N9779Y, was substantially damaged when an explosion occurred in the left wing during left engine start at the Louisville International Airport-Standiford Field (SDF), Louisville, Kentucky. The certificated private pilot, and all three passengers were not injured. Visual meteorological conditions prevailed for the personal flight, destined for Nashville, Tennessee. No flight plan was filed, and the flight was conducted under 14 CFR Part 91. According to the pilot, he landed at Louisville in order to pickup one additional passenger, before flying on to Nashville. Once on the ground, he was informed that he would have to pay a ramp and parking fee, or buy a minimum of 15 gallons of fuel. Even though the airplane would not require any additional fuel for the subsequent flight, the pilot opted to buy some fuel to avoid paying the fees. While the refueler was servicing the left auxiliary fuel tank, he allowed approximately 4 to 5 gallons to overflow. The refueler advised the pilot of his mistake, and said he would add a couple of extra gallons to the right wing to makeup for what had been spilled. The refueler completed servicing the airplane, and then repositioned the truck. The pilot examined the left wing for any evidence of residual fuel, and approximately 15 minutes after the spill had occurred, loaded his passengers in preparation for departure. The pilot started the left engine, and simultaneously an explosion occurred in the left wing. The engine did not start, but the pilot thought that it may have backfired at the time of the explosion. According to the refueler, he was instructed to add 7 1/2 gallons to each wing. He serviced the left wing first, and inadvertently spilled 1 to 2 gallons in the process. The spilled fuel ran down the wing, off the trailing edge, and "it appeared to be evaporating quickly" from the ground. He then serviced the right wing, and then parked the fuel truck approximately 150 feet from the airplane. Sometime thereafter, he heard an explosion, and saw flames on the lower side of the left wing. He retrieved the handheld fire extinguisher from the fuel truck, and proceeded to the airplane where he extinguished the fire. According to a Federal Aviation Administration inspector that examined the airplane, the left wing displayed damage consistent with an internal explosion, and the fuel cells were intact, full of fuel, and not leaking. The inspector added that he observed "very little fire damage" on or in the left wing.