For the last 40+ years, I have always washed FB papers, after a first rinse, with a HCA - in the 1970's, it was just washing soda, as that was recommended in books of the time and was both easy to get and very cheap!
Most recently it has been, as mentioned many times, good old sodium sulphite - using about a tablespoon full in about a litre of water.
In the 1970's the next step was to just leave the prints in the bath with a few inches of water, agitating them occasionally, and changing the water about four times.
Today, I use a Paterson upright print washer with a cradle and basically do the same thing as above. A first rinse of the prints, a soak in sodium sulphite followed by four changes of water with a little bit of agitation in between.
These two ways have given me stain free prints, even from the very beginning in the 1970's, of which I still have a few 10" x 8"s to prove it.
I was also given a posh FB washer, which is very heavy and I'm sure, words well, but I've only used it once or twice, then put it one side after I saw how much water it used...
Terry S