Heat is the key. I bought a mondo iron (the tip is about as big around
as a little finger) for $1 at a garage sale. It works perfectly and takes
to warm up. It also takes forever to cool down, which is what your're looking
Some other tips:
1) lightly tin the braid.
2) File with a halfround file, the area around
the opening for the solder, to expose
the base metal (hopefully, brass!)
3) Pre tin this area as well.
If I can find one more cheapie iron, I'm going to file the tip into a concave
to match the contour of the PL259 shell. Heat transfer should be enhanced
even more, in what will then be a dedicated cable ass'y iron.