Thanks Gary,
I did the welds as I went, partly because I didn't want to spend a week or 2 doing nothing but welds but also because as the tank came together and got heavier, it became more and more unwieldy for doing fine detail work.
For practice, I just sacrificed a piece of sheet styrene for use as a guinea pig - got 2 tries to get it right per sheet!
I also got a 1/35th elefant for practice, but I haven't put that together yet. I'll use the test pieces to try out my weathering techniques before I apply them to the tank.