Make a locomotive fuel tank out of bondo
Sometimes you need a larger fuel tank, or in the case of the Athearn C44-9W I got for parts, there was no fuel tank at all so I had to build it from scratch.
Bondo is a product that is easier to use than you would expect. Take a piece of card board from an old box and put some of the gray bondo in a glob with a plastic knife. Then put about a cap full of the red hardener next to it and mix the 2 together. It will turn pink.
You now have 3-5 minutes to glob it where you need it. I use tape barriers most of the time, but be warned that too much barrier and you can't get the bondo to go where you want it.
As soon as it starts to get a bit sandy looking when you mix it up on you card board that means time is up for spreading it.
Now you have 5-10 minutes where you can take a flat blade x-acto knife and trim it. You sculpt it as best you can carefully shaving it down.
At about 30 minutes in you can go to the disc sander and shape it like wood. Sometimes you wait a bit longer, but rarely do you need to wait an hour.
You can get it very smooth if you want using finer and finer grades of sand paper. Once you're done, you can paint it. I don't always use primer, but primer can be used to make the surface even smoother.