Deductions & credits

The thing is, fair market value is defined as what a reasonable person would pay for the same or similar item in similar condition.  If no one would buy the item in dirty condition (or contaminated, depending on what your work entails) then the value without cleaning would be zero, or at least less than the value when clean.   The donation value is the FMV at the time you donate it, regardless of what repairs or cleaning you have done in the mean time.