One is to apply a thin bead of putty, then rough it up a little to give it the texture of a weld. You can make it consistent by taping the area to be seamed with masking tape, leaving just the thin weld seam showing. Like a mask.
Another is to glue on a thin strip of either plastic rod or sheet styrene, again roughing it up once dry.
A third is to use a fine soldering iron or wood burning tip and melt the seam into the plastic.
All work relatively well. Good luck on whichever you choose.
I usually take a piece of heat stretched sprue and apply to the area where I want a weld seam and soften it with glue, I then use a very small electricians screwdriver where I have rounded the tip and make a weld seam texture.
I have also used the putty method around smaller details like these track hangers on a Tiger II turret.