When working on radiator or any soldering job and using Muriatic Acid, It is a good idea to use "Cut Acid". A tinner in the local plumbing shop taught me as a kid to dissolve pieces of zink fruit jar lids in the acid and then it tins much better and don't burn so bad when you get it on your skin. 50-50 is good alloy for solder. 50% tin and 50% Lead.