Reply by chrisstef

Ive seen a lot of commercial construction projects use a SIKA paint product as a rust inhibitor for metal decking as well as exposed rebar in concrete slabs.

http://usa.sika.com/en/group.html

From a little bit of reading it seems that the high alkalyds (sp) in concrete contribute to the rusting of metals. Id be willing to bet that the existing decking that’s rusting in your shop is galvanized as well. You may want to make sure that any new product you use will bond with the galvanized metal.

-- Its not a crack, its a casting imperfection.