Sheet metal?

I was cutting in patches on my Demon, I got some scrap piece of paint-loc from a sheet metal shop we use use at work and was having trouble welding them in. I talked to this old boy that chop tops on rat rods and told him about the problems I was having. His wisdom was, "I was welding new steel to old steel." He said to use old panels from the same time frame that my car was from. The steel from 30-40 years ago was made different then it is now and the two pieces will weld different. I had my old fenders and made patches with them. Night and day differents. Now that is all I use if I have old stuff to use. He knew what he was talking about.