Quote Originally Posted by oka374 View Post
I've always used a small file to clean the tip back to clean copper and then tinned them.
Was taught to do that as a PMG appentice a lifetime ago.
Iron tips these days are a copper core, an inter-layer (sometimes nickel from very hazy memory) and then an iron plating. The tip above the tinnable section is often chrome plated over the iron to stop solder wicking up there.

The problem with pure copper tips, or an iron tip where the plating has been destroyed revealing the copper is with time the copper dissolves in the solder leaving cavities in the tip and requires regular filing until eventually the tip is gone. A properly plated tip lasts considerably longer.

The last pure copper tips I had were for my Scope and Mini-Scope. At one point when I was doing a lot of work, the mini-scope was using a tip per fortnight. I upgraded to a temperature controlled Hakko with an iron plated tip and started changing them every 2 years.