At one time, I used Squadron putty, but I found its shelf life to be less than useful, and its application unsatisfactory, the latter perhaps due to my methods. It’s not the best for sanding, either.
Nowadays, I use Bondo’s Glazing and Spot Putty: it comes in a tube, is much cheaper than comparable hobby products, and seems to have an unlimited shelf life. Once applied, it hardens quickly (don’t apply it too heavily), and sands easily.
It’s available at any automotive supply store.
I first used it to repair a brass tender: a previous owner had attempted to alter the coal bunker, and started to cut-off the top of the sides, using a cut-off disc. It’s my guess that he started on the rather plain fireman’s side of the tender, but realised, when going to the engineer’s side, that the ladder detail was going to complicate the plan.
I soldered some sheet brass to the inside of the tender to close-off the inside of the cuts, then used some JB Kwik Weld to fill most of the depth of the cuts, making sure to not fill right up the the surface level of the sides.
The Bondo product comes in a tube, and upon opening it, I discovered a lot of liquid (the solvent) at the top of the tube. Usually, the process is to knead the closed tube to mix the ingredients, but instead, I simply stuck a small screwdriver into the tube, intending to stir them. Since the screwdriver wasn’t long enough to reach too far into the tube, I withdrew it, planning to use a longer screwdriver.
However, there was a nice blob of putty on the end of the little screwdriver, so I wiped it into the cut and smoothed it using an X-Acto chisel-type blade. It looked pretty good, so I simply continued with that method.
Here’s the fireman’s side of the tender after sanding…
