One hopes that the walnut blasting does not microscopically etch into the metal surface, giving carbon particles a better grip in the future! Only time will tell.
I would be diverting the breather without delay!
I think walnut shell blasting is one of the least invasive and damaging of the various blasting mediums Doug. It's particularly suitable for softer materials such as aluminium, GRP gel coat and even some woods. The medium has friability (new word for me) meaning it degrades on impact into powder/dust. First came across it during the vintage Lister diesel rebuild for my narrowboat propulsion. It was used to clean out cylinder heads without damaging mating surfaces.
