I've recently begun to take on projects like building kit amps, rewiring guitars, modding amps, biasing amps, etc. So I've developed soldering skills. I currently own a 40 watt Radio Shack pencil iron. Question: my tips are wearing out fast. Specifically, a notch begins to develop on the edge about 1/16 of an inch or so up from the tip. I keep the tip tinned between touches, and wipe it clean with the moist sponge on the soldering station after each touch then tin the tip before placing the iron on the stand. The notch begins to appear after 1 hour or so of use. BTW, I'm using Radio Shack tips, rosin core solder also. What could be causing this? I expect longer life from these tips.