A sharp high quality file is less prone to clogging in pewter, but
they all will to some degree. You can try rubbing chalk or Burlife
on before filing, but I find it’s more trouble than it’s worth.
A file cards teeth are generally too large to get in there and clean
them very well, I use a steel bristle scratch brush, similar to the
glass ones sold, to get most of it and a sharp mold cutting scalpel
blade to pop the rest out.