We have much easier access locally to Sika 221 than 512. Can pick it up locally easily and good stocks are kept too. I have to make an urgent repair to something and having used Sika 512 a couple of times was happy to us that, but it will create a 3-5 day delay. On the other hand it's possible to buy 221 locally tomorrow morning. Looked up the DataSheets for both and they are similar but not the same. Has anyone used both of these to have an opinion? BTW, have to seal galvanised sheets where the supports have moved and gaps have appeared.