I 've never tried hammer welding fenders, I usually try to find good ones, till now, I've always lucked out.However, I hammer-weld alot of cracks or rips in large damaged saws at work. I work wth large band saws in a sawmill. I usually start at the last part of the crack, not where the crack is exposed to the edge.I weld about 1/8 -1/4" of the crack at a time .Then while the metal is still red or yellow ,I hammer it to shape using a hammer and dolly to the required shape and thickness(approximately).I then repeat this process until I get to the edge.Depending on the length of the tear is how long it will take .It usually takes 1-5 minutes. You can do this with carbon steal as well, I have welded leaf springs before ,but it takes one one treatment:Annealling. Any ways ,you can practice like I did 17 years ago to learn my trade by taking a piece of scap sheet metal and cutting fake tears into it and welding it back to original.
After welding some people use a grinder to clean up the weld ,some people use a flapper wheel to dress the weld, I prefer the flapper. Caution, do not let the metal get hot while dressing the weld or it may crack.
I always anneal after all welding to stop the cracking ,in case the rod Im using has carbon in it.
To anneal, simply turn down the lights and bring the welded area to the dullest red glow,just so you can barely see it. Then try to hold that for a second and slowly take your torch away(take about a minute). Guess what, you are done. I know this will take practice, I had my share of cracks reopening. If you can master this,you'll save your self a lot of money and time.
After a while you should be able to weld frames as well ,dress the weld ,and it should be invisible if done correctly.
