Glues and filler for model kits: cements, superglue, putty and seam lines

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Choosing the right glue and filler makes the difference between a model that looks built and one that looks made. This guide covers the main types, how they work, and how to deal with the seam lines that appear wherever two halves meet.

Plastic cement: welding, not gluing

Most model kits are moulded in polystyrene, and polystyrene cement does not glue in the way people expect. The solvent softens and partially dissolves the surfaces of both parts, they fuse together, and the solvent evaporates. The result is a weld, which is strong but needs well-fitting parts because it fills almost no gaps. There are two main forms.

Tube cement is thick. It is easy to control on large joints but can string and ooze, and too much melts detail.
Liquid cement is thin and applied with a fine brush. Hold the parts together and touch the brush to the joint; capillary action draws the cement in. Thin liquid cements set quickly, and slower ones give more time to adjust.

Plastic cement only works on polystyrene and compatible plastics. It will not bond metal, resin, clear parts well, or most other plastics.

Superglue

Cyanoacrylate, usually called superglue or CA, bonds almost anything.

Thin CA wicks into joints like liquid cement.
Medium and thick CA stay where you put them and fill small gaps.
Accelerator makes CA set instantly, useful when holding small parts.

Superglue can fog clear parts, a white haze often called frosting or blooming, so avoid it near canopies and windows. It also bonds skin very quickly. Keep a debonder handy, and avoid getting it near eyes.

Glues for clear parts

Canopies and windows need a glue that dries clear and does not fog. Water-based glues sold for clear parts, or PVA, are popular. They clean up with water and dry transparent.

Epoxy

Two-part epoxy is slow but very strong and gap-filling. It suits heavy resin or metal parts and joints under load, such as undercarriage legs on large aircraft.

Filler and putty

Gaps happen even in good kits. Fillers include:

Solvent-based putty: classic modelling putty that shrinks slightly as it dries, so may need a second application.
Water-based or acrylic filler: easy to smooth with a damp cotton bud or brush before it sets, good for fine seams. It can be less hard once dry.
Two-part epoxy putty: mixed by hand, does not shrink, good for large gaps and sculpting.
Superglue as filler: a drop of CA, sometimes with talc or a special filler powder, sands to a hard, non-shrinking finish.
Stretched sprue or plastic card: for big gaps, fill most of the space with plastic first, then putty.

Dealing with seam lines

Wherever two halves meet there is a line. On a fuselage or a gun barrel, it is very visible under paint.

Dry fit first and check alignment. Sand mating surfaces flat if needed.
Glue with a little extra cement so a small bead of softened plastic squeezes out. Let it harden fully, ideally overnight.
Sand the seam flush with a sanding stick, working through progressively finer grades.
Fill any remaining gaps with putty or CA, then sand again.
Check under primer. Primer reveals seams you cannot see on bare plastic. Fix them before painting the colour coat.
Restore lost detail. Rescribe panel lines with a scriber and replace rivets if you sanded them away.

Masking tape either side of a seam protects surrounding detail from sanding.

Safety

Plastic cements, solvent-based putties and CA all give off vapours. Work with a window open or under extraction, keep lids closed when not in use, and read the safety data sheet for each product. Wear eye protection if there is any risk of a splash, and keep everything out of reach of children. Which glues and fillers do you swear by, and what has gone wrong? Share your experience or questions in the comments.