Filling seams on a plastic part is as easy as applying the putty, letting it dry, and sanding the area smooth. If you’ve already painted the parts, that adds a bit of a complication. Here’s how to handle it.
The dome for Bandai’s 1/12 scale R2-D2 came as halves with a lot of unusually shaped parts inside. Whether painting the parts before assembly was the right call, well, too late to worry about that now.

An obvious seam ran down both sides of the model’s dome — not at all what you’d want to see on a finished model. The spokes for the dome’s top insert would end up leaving unsightly seams, too.

Apply putty to both seams. Place tape alongside the seam to minimize the mess. Overfill the seam because the putty will shrink when it dries. Let the putty cure completely — 24 hours will do the trick.

Knock down the putty with a medium-grit sanding stick and then remove the tape. There’s no avoiding it: At this point, some paint is going to have to come off.

Mask anything you don’t want damaged by sanding or covered by the coming paint touch-ups. Sand the seams with progressively finer sanding sticks, eventually ending at 12000-grit. Make sure to feather the surrounding paint with the finer grits.

Airbrush black primer over the affected areas. Concentrate on the bare plastic and seam and get a smooth transition the surrounding paint. If the seam still shows up under primer, stop, let everything dry completely, add more filler, and then sand again. Patience, padawan!

With your primer coat done, airbrush your original color. Apply thin coats, building up an even finish. Concentrate on the seams and move outward. Metallic paints tend to make imperfections stand out, so make sure your primer coat is as smooth as you can get it.