WHILE EN ROUTE, THE PLT ENCOUNTERED HAZY WX AFTER CROSSING THE CONTINENTAL DIVIDE. NEAR MONTROSE, CO, HE DEVIATED ABOUT 50 MI NORTH TO AVOID ADVERSE WX. HE WAS UNABLE TO GET THRU, SO HE PROCEEDED BACK TO MONTROSE TO SKIRT THE SOUTHERN EDGE OF THE WX. HE STATED THAT NEAR DOVE CREEK 'A STORM MOVED IN' & THE WX 'STARTED LOOKING BAD.' HE INITIATED A CLIMB FROM 8500 FT MSL TO 12,500 FT MSL & WAS 'TRYING TO GET OUT OF THE AREA' WHEN SEVERE TURBULENCE WAS ENCOUNTERED. HE STATED THAT HE WAS CLIMBING THRU 11,500 AT 110 KTS WHEN 'ALL HELL BROKE LOOSE.' ACCORDING TO THE PLT, THE TURBULENCE FLIPPED THE ACFT INVERTED & FORCED IT INTO A CLOUD BANK. HE SAID THE ACFT WAS CARRIED DOWN & UP SEVERAL TIMES, LOSING & GAINING ABOUT 2000 FT EACH TIME. SUBSEQUENTLY, HE RECOVERED AT ABOUT 10,550 FT & AFTER APRX ANOTHER 30 SECONDS IN THE TURBULENCE, HE PROCEEDED TO HIS DESTINATION. AFTER LANDING SEVERAL 'RIPPLE' WERE FOUND IN THE WING SKIN. LATER, A MORE THOROUGH EXAM REVEALED EXTENSIVE INTERNAL DAMAGE.