A few weeks ago we had to leave the boat overnight for two nights in salt water. Unfortunately we did not have the time to swap the anodes over so we had fresh water anodes (magnesium) in salt water (should be zinc). We thought we we would be alright for the 48 hours but no, an electroplating action took place that coated the stainless props in a hard layer (I guess of magnesium of some sort). This would not clean off easily, even chipping away at it was difficult and slow. Therefore it was time to get aggressive. I had heard much about "Patio and Brick Cleaner" which is based on Hydrochloric Acid. A quick trip to Wickes resulted in a cheap container of cleaner.
I simply put the cleaner in a bowl and dipped in the props and Hey Presto! a great result.