I have a Sheaffer Imperial VIII which is leaking from under the nib, just below my finger. Is there a way to avoid this. I think ink gets under the nib and gets out when that part of the nib is touched either with finger or a tissue.

I had a similar problem with my Imperial VIII. There was an ink leak at the inlaid diamond. Ron Z at Main Street Pens used a solvent to seal the leak. I see no obvious sign of repair and now the pen writes without a problem.