Thin or thick plastic cement — which one do I need?

Two products that look identical on the shelf and behave completely differently. Which one welds a seam invisibly, which one buys you time, and where superglue and PVA belong instead.

Difficulty: beginner
Time: 10 min
Read: 4 min
Safety: caution

Short answer

Thin cement is watery, applied by brushing it into a joint you are already holding closed, and drawn through by capillary action — it is the default for almost every plastic-to-plastic joint. Thick cement is a gel applied to one surface before assembly, for joints you need time to align or that need gap filling. Neither works on resin, metal or clear parts.

Both products melt polystyrene and weld the two parts into one piece, which is why they are far stronger than any adhesive. The difference is entirely in how they are applied, and choosing the wrong one is the reason people end up with fingerprints melted into a fuselage or a joint that opened up while the glue was still soft.

Safety

Plastic cement is a solvent — usually a mixture based on butanone, acetone or dichloromethane — and the vapour is heavier than air, accumulates in a still room and causes headache and dizziness long before you notice a smell problem. Work with a window open. Keep it well away from any flame, including a pilot light. Some formulations are classed as harmful by inhalation and the label states which; read it once rather than never.

What you'll need

Thin liquid cement with a fine brush applicator
Thick or gel cement in a tube (optional)
Cyanoacrylate for metal, resin and photo-etch (optional)
PVA or a specific clear-part adhesive for canopies (optional)

Hiding a seam on a model, in cross-section, stage by stage

Step by step

Use thin cement as the default.Hold the two parts together in their final position, touch the loaded brush to the outside of the seam, and the cement runs into the joint by capillary action along the whole length. The joint welds while you are still holding it.
Press the parts together firmly for a few seconds.You want a fine bead of softened plastic to squeeze out of the seam. That bead is melted plastic, not glue, so it sands to the same hardness as the part and the seam disappears completely.
Use thick cement where you need alignment time.A large fuselage half, a wing root, anything that has to be clamped and adjusted. Gel does not run, so it stays where it is put and gives a working time of a minute or two rather than seconds.
Use thick cement where a joint has a gap.Gel has body, so it fills a small gap and welds across it. Thin cement runs out of a gap and welds nothing.
Never use either on clear parts.Both solvents craze and fog clear polystyrene on contact, and the vapour alone will fog a canopy if it is nearby while a joint cures. Use PVA-type white glue or a dedicated clear-part adhesive, which dries clear and does not attack the plastic.
Use cyanoacrylate for anything that is not polystyrene.Resin, white metal, photo-etched brass and most 3D-printed parts do not dissolve, so a solvent cement does nothing at all. Superglue sticks them; a slow-setting gel type gives you time to place them.
Keep the applicator brush clean.Thin cement dissolves the softened plastic it collects and the brush turns into a gummy stub. Wipe it on the neck of the bottle before every application.
Let a joint cure before you sand it.The plastic either side of a fresh weld is soft for far longer than the surface suggests — often overnight for a heavy joint. Sanding a soft seam dishes it, and the hollow only shows up under primer.

Tips

Thin cement applied to a joint you are not holding closed does nothing useful. Assemble first, then cement.
A tiny amount of thin cement brushed along a mould seam softens it enough to scrape away cleanly, which is quicker than sanding.
Buy the type with the metal needle applicator if you can. It puts cement exactly where you want it and nowhere else.

Common mistakes

Using thick cement as a general-purpose glue — Gel squeezes out into a ridge that has to be sanded off, and because it is thick it takes far longer to cure, so joints move after you think they are set.
Applying cement to both surfaces before assembly — That is the technique for an adhesive, not a solvent. With cement it doubles the amount of softened plastic, melts the surface detail and guarantees squeeze-out on the visible face.
Holding a freshly cemented part in your fingers — The plastic is soft. Fingerprints press permanently into it, and they show up under paint far more than they do on bare grey styrene.

If it doesn't work

The joint keeps opening as it dries

Cause: Thin cement in a joint that was not held closed, or a gap the cement ran out of — Fix: Clamp or tape the joint closed, re-run thin cement into it, and hold. If there is a real gap, use gel or a strip of plastic card as a shim.

Melted crater where you touched the part

Cause: Too much cement, or handling before cure — Fix: Let it cure completely, fill with model filler slightly proud, sand flush and re-prime.

The canopy has gone milky

Cause: Solvent cement or its vapour reached clear plastic — Fix: Light fogging sometimes polishes out with plastic polish or a dip in floor-polish type acrylic. Heavy crazing is permanent — replacement clear parts are often available.

Nothing sticks at all

Cause: The part is resin, metal or a non-styrene plastic — Fix: Use cyanoacrylate or two-part epoxy. Solvent cement only works on plastics it can dissolve.

Questions people ask

Can I use superglue instead of plastic cement?

You can, and it will hold, but it is a weaker joint on polystyrene because it sticks rather than welds, and it cures hard and brittle so it sands differently from the plastic either side. Use it for metal, resin and photo-etch, and cement for plastic.

Does plastic cement go off?

Yes. The solvent evaporates through the cap over months, and what is left is thicker and slower. If it strings, gels or stops running into joints, replace it — it is a cheap consumable and old cement causes far more damage than it saves.

What about extra-thin cement with dichloromethane?

It is the most capillary-active and the fastest, which makes it excellent for fine seams and unforgiving of a slip. It is also the most volatile, so ventilation matters more with it, not less.