Etching problem in 304 stainless steel in ferric chloride solution

I am encountering a smut black residue while etching 304 stainless in 42 baumé ferric chloride ⇦ on eBay or Amazon [affil link] solution at 150 F. The residue is easily wiped off but since the part is subsequently plated, it a real annoyance. Initially this did not occur but now consistently happening. Does anyone know the cause or, more importantly, a cure to prevent it. Because of the fine detail in the etching pattern, mechanically wiping it off is not a practical solution.

A quick swill in 50 gpl Chromic Acid will clean out the smut.

Is the ferric chloride old or new? You say it is 42 baumé, but is that the original density or the current one? The problem may be caused by an exhausted solution.