ammonium persulfate. no stains. Sodium persulfate also works. You could even use hydrochloric acid though it's slower and you want speed with etching.Unfortunately, it needs a strong solution of h2o2 to work well. see this instructables

 

Oh god after battling the buggy software for a few hours I got this, the software is so annoying. sometimes there's a connection and the software won't register it. specially when using copper area or two layers.

 

I use hydrogen peroxide and hydrochloric acid (also sold as muriatic acid for maintaining ph in swimming pools). I used to loos a lot of shirts to ferric chloride but after switching to this solution I have not lost a shirt or stained a sink (because now I do it all out of doors

https://mad-science.wonderhowto.com...utomatically-improves-after-each-use-0135192/