Ceramic building seams

Automotive glazes. 3M Acryl-White. Or you might try using some Plaster of Paris or some of the Sculpy like clays that dry hard.

Like Hawkeye said, plaster of paris or something like that. I recently did a ceramic church ruin and I used the spackling for drywall you can find at hardware stores. I ran it into the seams with a toothpick and now you can't tell there was even a seam. The stuff is pretty cheap too for the amount you get.

White glue or putty...

I fill it with Plaster of Paris. Put it on dry and then moisten, or use a small spatula to spread wet, mixed plaster across the seam. Sand when dry to feather it, even it out. I usually wait a day or so.