Volcanic ash does not affect VFR aircraft....
In June 1991, Mount Pinatubo erupted in the Philippines. The volcanic ash drifted across to Brunei where I was flying the S61s based there. One of my colleagues experienced compressor stall as he applied power at the end of his approach at our base. On inspection, it was found that substantial build up was occuring to the compressor blades which required the implementation of 2 compressor washes per day to prevent further engine problems.
It was very rare that any of the helicopters there operated IFR, so I think the above statement isn't exactly true........