This can sometimes happen and its nothing to worry about!
This is caused by hop oils and fatty proteins from the yeast and malt and can occur on any stainless steel fermenters, not just Grainfather units.
We suggest;
- A hot soak with Grainfather High Performance Cleaner or PBW for 30 minutes
- And a bit of elbow grease with a sponge or dish brush, this will remove the residue and have it up and running ready for your next batch!