Do your self a favor and buy a small book on the different types of weld "Beads", There are several different types used in welding a tank together. The most common mistake modelers make is trying too make what would be a huge weld in real scale look like one pass of weld. this is not correct. Very large welds which is what you will be modeling usually require multiple passes called "Beads" a series on narrow welds built on top of each other. This prevents the base metal from over heating and possibly having the weld crack.  Lay the putty i the seem and take a pin and drag a series of lines in the putty. Then smooth them slight;ly with a wet finger. It should look like a bunch of stripes, NOT one BIG Weaved weld.