Homebrew PCBs

> Best thing would be to just use as little as possible and dispose of
> it after use. 

Just read both manual and MSDS
http://www.mgchemicals.com/downloads/pdf/specsheets/421.pdf
http://www.mgchemicals.com/msds/english/liquid/421-liquid.pdf
http://www.mgchemicals.com/products/421.html 

It says:
Clean the board, either by acid dip, degrease or both
5 minutes on that thing
HOT water rinse

It's a fluoroboric mix, which is a strong acid, in it the tin doesn't
oxidize much in air, it can be left in the tray, and still works for
long. That's why is a good product that worth the money. Besides the
'boric' name it's not more harmful than other strong acid. Disposal
shouldn't make such doubts in a so long lasting product. The white
deposit should be stannous fluoroborate and not oxidized tin like other
inferior mixes, someone said it get plated with use. The thing is smelly
thanks to thiourea, the name says it all, and it's a suspected
carcinogen. Use gloves and don't share bottles/trays for it's use with
other household things. There is no harmful gases produced, other than
smell a chemical is always bad. But every immersion tin on copper mixes
with msds found by google uses thiourea, so no other option for that.

This isn't a electroless chemistry, is a immersion chemistry. Works by
the surface transactions between copper and tin on the acid, in
equilibrium. Thiourea produces a strong complex with copper, so forces
the reaction to one side. Will go until surface cover with tin. Any
oxide or grease on surface voids the reaction. A good clean surface
makes a instantaneous beautiful finish, not a gray dark or black finish
slowly made. Thiourea makes a fatty thing with copper left on the tin
surface so it should be used warm-hot water to clean pcb after use. If
used corrosive liquids on the so thin tin surface, like some flux, the
tin is like washed away.

Questions ask mgchemicals.