Here is the best way that I have found to deal with seams. It does not eliminate sanding 100% but it sure helps and saves a heck of a lot of detail.
This is a five-step process that moves along at about three inches at a time.
Step 1: Cut thin strips of masking tape and place along both sides of the seam leaving about a three-millimeter gap.
Step 2: Using a trowling tool (I use a potters detail tool for clay) lay on a coating of white putty.
Step 3: Immediately remove the tape pulling it away from the wet putty.
Step 4: Use a good quality cotton swab (I'm using a Q-Tip) dipped in acetone-based fingernail polish remover and wipe down the putty.
Smooth out the seam. This will greatly reduce the amount of sanding required.
Step 5: After the putty has had ample time to dry, sand the seams with a fine sand paper.
And this is what the finished product looks like...
