6. Fill and smooth
If you stand in an airport terminal and look out at the planes, you’re not likely to see one with a longitudinal seam on top. Effective seam-filling is key to a realistic model.
The best way to eliminate a seam is to sand (or carefully slice off) the plastic that oozed out when you glued it. Sometimes that’s enough to conceal a seam. But a long seam like a fuselage join will probably have gaps where the seam is not filled.
Modelers all have their favorite fillers. Many use super glue, either the standard thin viscosity (great for running along fuselage seams) or gap-filling (for larger gaps). If you fill with super glue, sand it before it cures. Otherwise, it’s tougher than the plastic around it.
A solvent-based filler, Squadron Green Putty can be scooped and smoothed before it hardens. However, it shrinks when it dries. Also, it stinks to high heaven and, in fact, contains toluene, a toxic irritant.
Two-part epoxy putty, such as Milliput, has more body and is easy to sculpt before it cures. However, after curing is much tougher to sand smooth. A water-soluble counterpart is Apoxie Sculpt two-part putty.
Then there are several water-based or relatively inert fillers, such as Deluxe Materials Perfect Plastic, which work well. And in the FineScale Modeler mail basket are several do-it-yourself-type accounts from people claiming success with wall-joint compound, spackle, even Liquid Paper correction fluid.
Whatever you choose for a filler, the idea is to fill the gap or imperfection and, once the material is in an optimal state, sand it smooth so the repair disappears underneath a coat of paint. Padded sanding sticks work well for this, riding on the high spots without damaging the surrounding “good” plastic. As you work off the excess filler, use progressively finer grits so that when you are finished no scratches are visible.
Surface treatments such as Gunze Sangyo Mr. Surfacer can take you to the last step by filling in fine scratches and sanding smooth for the desired finish.