Seam line

A seam line is what remains after two halves of a part, like the left and right sides of a fuselage or hull, are glued together. Even a well-fitting kit rarely produces a perfectly invisible join straight out of the box, so most builds include some stage of filling, sanding, and re-scribing lost detail along these lines before paint goes on, especially on rounded surfaces where a seam is easiest to spot.

Dealing with a seam usually starts with test fitting the halves dry to see how much of a gap needs filling, since a tight-fitting kit may only need light sanding while a looser one needs putty first. After the glue has fully cured, the seam is sanded in stages from a coarser to a finer grit, and any panel lines or rivets that were sanded away in the process are scribed back in before priming.

The most common mistake is sanding or priming before the glue has fully cured, which can trap solvent under the surface and cause the seam to sink back in later, sometimes days after the model looked finished. Rushing straight from glue to paint without a proper cure and check under primer is the single biggest cause of seam lines reappearing on an otherwise finished build.