Filling and Sanding Seams So They Disappear

The joint along a fuselage spine, turret shell, or vehicle hull side is easy to miss until primer puts a gray line around it. Most seam work is decided before the putty comes out. The cleaner the fit and glue joint, the less filler has to do later.

Putty is for what remains after the plastic has been joined. If it carries the seam by itself, it can shrink, chip, or leave a soft edge under sanding.

Glue, Cure, Fill, Sand, Prime

• Dry-fit the parts first. Tape or clamp the halves together and look for steps, pin marks, warped edges, or tabs that hold the joint open. Scrape mating surfaces clean before adding glue. A small ridge inside the joint can create a seam outside the joint.
• Glue for a plastic bead. Apply plastic cement to the mating surfaces and press the parts together so a slight bead of softened plastic squeezes out along the seam. That bead is kit plastic welding to itself, not filler sitting on top.
• Hold the joint while it sets. Use tape, clamps, rubber bands, or fingers, depending on the shape. Keep the edges level. A gap is easier to fill than a step between two surfaces.
• Let the glue cure. Do not sand the bead while the plastic is still soft. It will tear, smear, or sink later. Follow the directions for the cement, and give heavy joints more time than thin edges.
• Trim and sand the bead. Shave raised plastic carefully with a blade if needed, then sand the seam. Start only as coarse as the job requires. Move to finer grits as the ridge disappears. A sanding sponge helps on curved surfaces; the Tamiya 600 Grit Sanding Sponge Sheet and Tamiya Sanding Sponge Sheet 1000 Grit are useful bench grades for this stage.
• Putty only what remains. If a hairline gap or small low spot is still visible, apply a thin layer of putty. Tamiya Putty Basic Type or Vallejo Plastic Putty can handle this sort of cleanup. Use less than seems necessary, then add more after it dries if the seam still shows.
• Prime the area. A light primer coat turns guesswork into evidence. Look across the surface under a lamp. If the line is still there, sand or fill that spot again. Repeat only where the primer shows a problem.

Mistakes That Keep the Line Coming Back

The common mistake is sanding too soon. Soft glue and soft putty both look smooth for a moment, then shrink or pull away after paint. Let each material cure before judging the surface.

Another problem is using putty to bridge a wide, loose joint. Fix the fit first. If the parts can move, filler can crack when the model is handled.

Pressing too hard with sandpaper can flatten round parts and leave a trough beside the seam. Let the abrasive cut. On aircraft fuselages, gun barrels, drop tanks, and curved armor castings, use a sanding sponge or wrap paper around a shape that matches the surface.

Primer is an inspection coat here. Bare plastic, glue marks, and putty all reflect light differently. Primer makes them one color, which shows the remaining ridge or hollow before the final paint goes on.