THE PILOT SAID HE WAS FLYING SOME FAMILY MEMBERS ON A TOUR OF THE SIERRA NEVADA MOUNTAIN RANGE FOOT HILLS WHEN THE FLIGHT ENCOUNTERED SOME RAPIDLY BUILDING CUMULUS CLOUDS WHILE CRUISING AT AN ALTITUDE OF ABOUT 11,500 FEET MSL. THE PILOT SAID HE WAS NAVIGATING AROUND THE BUILD UPS ATTEMPTING TO MAINTAIN VFR CONDITIONS WHEN THE CLOUDS SURROUNDED THE AIRCRAFT. THE FLIGHT INADVERTENTLY ENTERED ONE OF THE LARGE CLOUDS AND THE PILOT SAID THE AIRCRAFT ENDED UP IN A HIGH SPEED SPIRAL DIVE AT AN AIRSPEED IN EXCESS OF ABOUT 190 KNOTS. THE PILOT FURTHER STATED THAT HE, WITH THE HELP OF HIS SON, 'STARTED PULLING BACK ON THE WHEEL' AND RECOVERED CONTROL OF THE AIRCRAFT. THE FLIGHT THEN RETURNED TO ITS HOME BASE AND LANDED. EXAMINATION OF THE AIRCRAFT BY FAA INSPECTORS REVEALED THAT BOTH WINGS HAD MAJOR BUCKLES AT THE PRODUCTION JOINTS BETWEEN THE INBOARD AND OUTBOARD WING PANELS, WITH THE OUTBOARD PANELS EXHIBITING AN UPWARD PERMANENT SET. IN ADDITION, THE FUSELAGE IMMEDIATELY AFT OF THE FIREWALL AND FORWARD OF THE EMPENNAGE EXHIBITED AREAS OF SKIN BUCKLING.