A traditional and time proven seasoning.
The makers of this product are so confident that they actually mention " We Gaurantee to make any bag airtight thats not actually burst"... A hefty statement, but a high quality seasoning product to back it up.
Just heat the tin up in a pot of water, pour into your pipe bag working it into the skin and stitching, let it drain, and your away.
Airtight Seasoning available in 250ml