How do I do a panel line wash on a model?

A wash that flows into the recesses and comes back off the flat surfaces makes moulded detail readable. The gloss coat that makes it possible, the removal that makes it look right, and the acrylic alternative.

Difficulty: intermediate
Time: 45 min
Read: 6 min
Safety: caution

Short answer

Gloss coat the model first. Apply a thin enamel or oil wash, letting capillary action draw it along the panel lines rather than painting over the whole surface. Let it set for a while, then wipe the flat areas clean with a cotton bud damped in white spirit, always working in one direction along the airflow or the panel. Seal with a matt coat.

Do this first

Can I paint enamel over acrylic, or acrylic over enamel?

Kit makers mould panel lines and rivets as fine recesses, and paint reduces them almost to nothing. A wash puts a darker tone back into those recesses so the eye reads the shape. The difference between one that works and one that looks like a dirty model is entirely in the removal stage, which is where most of the time goes.

Safety

Enamel and oil washes are thinned and removed with white spirit, which gives off vapour and is absorbed through skin. Ventilate, and wear nitrile gloves for the removal stage, which involves a lot of solvent on cotton buds. Cloths and buds soaked in oil-based products can self-heat as they oxidise — lay them flat to dry before binning them rather than balling them up.

What you'll need

Gloss clear coat over the base colours
An enamel panel line wash, or artist oils and white spirit
Cotton buds and a fine brush
White spirit or a dedicated enamel thinner
Matt or satin clear coat for the finish

Which paint can go over which, as a layer stack

Step by step

Gloss coat everything first and let it cure hard.This is non-negotiable. On a matt surface the wash soaks into the paint and stains it permanently, so it cannot be removed from the flat areas and the panel line effect never appears. Gloss also lets the wash run freely along the line.
Choose the wash colour by the base colour, not by realism.Black over a light grey aircraft reads as cartoonish. A dark grey or a brown a couple of steps darker than the base colour is usually right. On very dark subjects, a lighter wash can be more effective than a darker one.
Touch the loaded brush to the line, do not paint the panel.Put a small amount at one end of a recess and let capillary action draw it along. The wash finds the line for you. Painting the whole surface just makes more to clean off.
Let it set, but do not let it fully cure.Enamel washes want somewhere in the region of fifteen minutes to an hour depending on the product and the room. Too soon and wiping pulls it out of the recess; too late and it will not come off the flat areas without heavy solvent.
Remove with a barely damp cotton bud in one direction.Damp with white spirit, not wet. Wipe in the direction of airflow, or along the panel, consistently. Directional streaking looks deliberate; random circular wiping looks like dirt.
Change the bud constantly.A loaded bud redeposits what it has just removed. Two dozen buds on a model is normal. This is the stage that takes the time and it is the stage that decides the result.
Leave a little behind deliberately.A trace of wash trailing back from a panel line, or slightly heavier around a hatch, reads as use. A model wiped to perfect cleanliness looks like the wash was never there.
Seal with a matt or satin coat.It locks the wash in, kills the gloss, and unifies the sheen so the decals, the wash and the base coat read as one surface.

Tips

Artist oils thinned with white spirit make an excellent home-made wash, cost very little for a huge quantity, and stay workable for hours — which is a genuine advantage over commercial washes that set fast.
On a very fine recess, a wash that is too thick will not travel. Thin it further and apply twice rather than trying to push a thick wash along a line.
Test the wash on the underside or on a spare part first. Some commercial washes are much stronger than others and some contain solvents hot enough to affect the gloss coat.

Common mistakes

Washing over a matt finish — The wash soaks in and stains the paint. The flat areas end up permanently grey, the recesses barely darker than the surroundings, and the only fix is repainting the panel.
Using pure black on everything — Real panel lines are shadow, not ink. Black reads as a drawn outline and makes the model look like a toy. A dark version of the base colour reads as depth.
Wiping with a solvent-soaked bud — It floods the recess and pulls the wash straight back out along with the panel-line effect, and it can attack the gloss coat. Barely damp is the correct amount.

If it doesn't work

The wash will not come off the flat surfaces

Cause: Left too long, or applied over matt paint — Fix: More solvent and patience if there is a gloss coat under it. If the surface was matt, there is no recovery short of repainting the panel.

The wash pulled straight back out of the line

Cause: Wiped too soon, or the bud was too wet — Fix: Let the next application set longer and use a drier bud. Re-run the wash into the line and try again.

The wash will not travel along the recess

Cause: Too thick, or the recess is too shallow — Fix: Thin it further. On a very shallow line, deepen it with a scriber before painting next time.

The gloss coat crazed where the wash sat

Cause: The wash solvent was too aggressive for that clear coat — Fix: Test on a spare part first. Use a milder thinner such as odourless white spirit, or switch to an acrylic wash which uses water.

The whole model looks grubby rather than detailed

Cause: Too much wash left on the flat areas, or too dark a colour — Fix: Take more off, and choose a wash closer to the base colour. Less is almost always better.

Alternatives

Acrylic wash: Water-based, so it can be used over acrylic paint without an enamel involved. Much less forgiving because it dries fast and does not re-wet easily, so removal has to be quick.
Panel line accent pens: Fine-tipped applicators loaded with enamel wash. Convenient, controllable and good on a small model. Expensive per millilitre and limited in colour.
Pre-shading before the colour coat: Darkening the panel lines with an airbrush before the base colour and then spraying the colour thinly over it. Subtle and very effective, and it needs an airbrush and more skill.
Pencil: A soft graphite or coloured pencil run along a recess. Crude but instant, useful on very small models, and it needs sealing or it rubs off.

Questions people ask

Enamel or acrylic wash?

Enamel over an acrylic base is the standard because white spirit does not touch the cured acrylic underneath, so removal is safe and unhurried. Acrylic washes work but you are using the same solvent as the base coat, which leaves no margin.

How long should I wait before wiping?

It depends on the product, the thinner and the room, so test on a small area. As a working range, somewhere between fifteen minutes and an hour for enamel. The point you want is when the wash has stopped flowing but has not hardened.

Does this work on miniatures too?

The principle does, but miniatures normally use an all-over shade rather than a panel line wash, because they have no panel lines. See the guide on painting a miniature for how shading works there.