From the source:
An enjoyable job for Marine plane captains of VMA-311 as they wash grime and dirt from their A4E Skyhawks. The washing is necessary to prevent corrosion and clogging of moving parts on the exterior of the plane. Lance Corporal William E. Lysle (Burbank, Calif.), left, and Lance Corporal Porter R. Tkachuk (Boston, Mass.), mix another batch of soap to clean the aircraft.