Just FYI....the acid from HD or Lowes is junk.  They call it "green" cuz they pretend that it is safer.  That is not the case - it is just weaker.  If you read the MSDS it is only something like 10% and it costs $7 or 8 a bottle.  If you go to a pool supply place (look around, there are tons), you can get it in 30% concentrations for like $10 a gallon.  You use less and it goes further for cheaper.  Just be careful with the stuff....it is pretty nasty.