Do I need to prime a model, and which primer should I use?

Priming is often described as making paint stick, and that is the least of what it does. Most model paints adhere adequately to clean styrene. What primer actually gives you is a uniform surface, a uniform colour to paint over, and — crucially — visibility of every seam, scratch and fingerprint at the last moment where they can still be dealt with.

Safety

What you'll need

• Primer — aerosol, airbrush or brush-on
• Washing-up liquid and warm water
• Somewhere outdoors or extracted to spray
• A handle or stick to hold the part (optional)

Step by step

1. Wash the parts first.Mould release agent from the factory and grease from your hands both stop primer keying. Warm water, a drop of washing-up liquid, a soft brush, then a thorough rinse and full drying. This one step prevents most adhesion complaints.
2. Decide whether you need it at all.Bare untouched plastic, one colour, a well-pigmented paint — you can go straight on. Sanded or filled surfaces, mixed materials, bright reds and yellows, metallics, or anything that has to be masked later — prime.
3. Choose the colour deliberately.Mid grey is the default because it shows flaws best and is neutral under most colours. White under yellows, oranges and reds, which are transparent and look muddy over grey. Black where you want depth and deep shadow. Oxide red is a traditional base under armour and rust.
4. Match the primer to the material.Acrylic or lacquer primers on polystyrene are both fine, but a hot lacquer primer can craze thin clear parts and some soft plastics. Resin and metal want a primer that etches or grips rather than one formulated for styrene.
5. Spray in thin coats from a hand span away.Two or three light passes rather than one wet one. A thick primer coat is worse than none — it drowns the moulded detail the kit maker paid for, which is exactly what you were trying to preserve.
6. Check under a raking light before you go further.This is the moment primer earns its cost. Look along the surface with the light almost parallel. Seams, scratches, sink marks and fingerprints all show as shadows. Fix them now, not after the colour coat.
7. Let it cure, not just dry.Touch dry is quick; hard enough to mask over or handle is much longer, often overnight. Masking over half-cured primer lifts it in strips.
8. Consider brushing primer on small parts.Brush-on acrylic primers exist and work well on a handful of small components, on a part you do not want to unmount, or when you have nowhere to spray. They are thicker, so thin them and use two coats.

Tips

• Primer straight from a cold garage will spray rough whatever the weather. Stand the can in hand-hot water for a few minutes first, never on a radiator or in a microwave.
• Shake for the full time on the label, which is usually far longer than people do. Under-shaken primer sprays out as clear medium first and pigment later.
• Mount parts on cocktail sticks pushed into a block of foam. You can then spray all round without handling, and set them down to cure without touching a wet surface.

Common mistakes

• One thick coat instead of two thin ones — It fills panel lines, softens rivet detail and takes far longer to cure, and any masking done over it later will pull it off in sheets.
• Priming a model you have handled with bare hands — Skin oil is an effective release agent. The primer looks fine, and then the paint over it lifts in exactly the shape of a thumbprint.
• Spraying indoors with a window open — An open window is not ventilation for an aerosol. The overspray settles on every surface in the room and the vapour concentration near you is unchanged.

If it doesn't work

Questions people ask