It depends on what sort of weld you are modeling.
I recently did a Tamiya M4 Sherman. In real-life, the welds on the hull should be flush to slightly proud of flush, whereas Tamiya molded them as being sunken. What I did was to carefully fill in the joint with putty. I used Tamiya basic gray putty since it had more working time than Squadron green. I then used a thick paint and a fine (No.0) brush to lay on a heavy bead of paint to build it up a bit. Then as the paint was drying, I stroked down the drying paint bead, to give some texture and suggest weld beads. SInce some Shermans do have welds that are built from multiple, parallel passes, lengthwise, this gave a decent scale effect. There were also some hull welds that were not really visible on the model. For those, I used the paint technique, but didn't have to pre-fill with putty.
Another technique that I have heard mentioned, although I've never done it myself, is to take stretched sprue, soften it with liquid cement, and use an Exacto blade or similar tool to shape it as needed on the model.
Regardless of what technique you use, keep a colse eye on the size of what you are trying to reproduce so that you don't make your welds overscale. For more replies on different techniques, post your question in the Armor Forum. This is something where people can be quite creative.