Decided to carry this over from the "stainless" thread ... we all deal with various chemical compounds every day... some benign some downright scary... on the subject of cleaning off discoloration from stainless welds... the "pickling pastes" are pretty common.. will not rehash my concerns.. that is back at the other thread... but it did occur that while most of us know what a MSDS is... some may not and others may not be aware how to find them... there are some excellent free databases for Material Safety Data Sheets on the net..
here is a link to one of them..

http://www.msdssearch.com/DBLinksN.htm

if you end up working with some chemical compound that you are unsure of... it is cheap self protection to make sure that it will not tear you up.. as some of those pastes can...
take care
Heiti