Painting a Mini 4WD Body
A Mini 4WD body is the one part every technical inspector actually looks at, and a bad paint job can ruin a shell even when the chassis underneath is perfect. Painting is also where ABS and polycarbonate shells stop being the same job: one is coated on the outside like any plastic kit, the other on the inside in reverse colour order — mixing up the two is the most common reason a first attempt looks worse than the bare shell did. This guide covers what the rules require of a painted body, the two Tamiya spray lines built for the job, and the prep, technique and curing differences that keep a finish on the car instead of flaking off it.
ABS and polycarbonate need opposite techniques
The shell that ships pre-formed in most kits is rigid ABS, trimmed at the sprue gates and painted on the outside — primer, then colour coats, in the order you'd expect on any styrene model. Tamiya's unformed polycarbonate ("clear body") shells work the opposite way: a thin, semi-transparent sheet is cut along printed guide lines with a hobby knife or curved scissors, then sprayed from the inside. Because the colour goes on inside-out, the order reverses too — detail colours and any pattern go on first, against the clear plastic, and a solid backing colour goes on last to seal and back the scheme, since that final coat is what shows through once the shell is mounted the right way round. Reverse the order and the backing coat buries every detail colour under it. For how much lighter a cut polycarbonate shell typically ends up versus the ABS original, see the bodies guide.
What the official regulations require
Tamiya's official regulations set one plain requirement for every class: bodies "should have stickers applied or be painted, and must be securely attached to the chassis." Detachment mid-run means retirement from that race. The regulations also name what doesn't count as a body: a homemade shell, a cut-off clear-body offcut, or a wing with no body — and a particularly small body, or one hard to inspect for paint or stickers, can be turned away at the table.
| Rule | Requirement |
|---|---|
| Body finish | Painted or stickered, securely attached |
| Detachment mid-race | Retirement from that race |
| Not recognised as a body | Homemade shell; cut clear-body offcut; wing with no body |
| Length / width / height | Under 165mm / 105mm / 70mm |
| Minimum weight (incl. motor, batteries) | 90g |
Stock Class narrows cutting further: only shaving material down or removing a genuinely obstructing section is permitted, splitting a body into pieces is not, and a polycarbonate shell must still be cut along the kit's own printed lines rather than reshaped freehand. Confirm current wording against the official regulations and Stock Class rules before an event — organiser rules and inspection prevail over any guide, this one included. The official rules and tech inspection checklist covers the rest of the pre-race check.
Two spray lines, and which shell each one goes on
Tamiya sells two dedicated lines for this. TS Color Spray is the general-purpose lacquer for painted plastic surfaces, including a stock ABS body's exterior, currently numbered up to TS-102. PS Spray for Polycarbonate is formulated for the flexible, solvent-sensitive inside of a clear body, currently numbered up to PS-63. Standard TS lacquer can crack or craze polycarbonate instead of bonding to it, so the two aren't interchangeable even when the colours look identical.
| Spray line | Numbering | Painted surface |
|---|---|---|
| TS Color Spray | 85000s, up to TS-102 | Outside of a rigid ABS shell |
| PS Spray for Polycarbonate | 86000s, up to PS-63 | Inside of a clear, unformed shell |
Preparation matters more than the paint
Every shell carries mould-release residue from manufacturing, and paint applied over it lifts later no matter how carefully it went on. Wash the shell in mild soapy water and dry it fully first. On ABS, a light key with fine abrasive paper gives primer grip; on polycarbonate, skip abrasives and wipe the inside with a lint-free cloth just before spraying, since trapped dust shows as a permanent fleck on track. Dry-fit the cut shell over the chassis before the first coat — trimming after painting risks chipping a finish already committed to.
Thin coats, then cure in the dry
Several light coats beat one heavy one on either material. A heavy, wet coat runs, takes far longer to cure under its own skin, and adds real weight where every gram counts against the 90g floor — see the weight and balance guide for where spare weight is better spent. Humid air extends cure time and raises the risk of blooming, a hazy patch where moisture gets trapped in the drying lacquer, so paint in the driest part of the day and give the shell longer than the can suggests before handling it. The Malaysia climate care guide covers the same humidity problem across the rest of the car.
Masking for multi-colour schemes
Two-tone schemes build in reverse on a polycarbonate shell: the colour that sits on top once the car is right way up goes down first, with the surrounding area masked and sprayed after. Low-tack tape and a sharp blade keep a hard edge without lifting the coat underneath — press it down firmly before the next colour, and pull it while still fresh, not fully cured. See the custom decals guide for combining painted sections with decal work, and the body trimming guide for a clean cut before paint goes near it.
Checklist
- Wash the shell and let it dry fully before any primer or colour coat.
- ABS gets exterior colour, polycarbonate gets interior backspray — never the reverse.
- Use TS Color Spray on ABS and PS Spray for Polycarbonate on clear shells; don't swap them.
- Paint detail and pattern colours first on polycarbonate, backing colour last.
- Apply several thin coats and cure longer than the can suggests before handling.
- Recheck that the finished body is securely attached before racing.
Frequently asked questions
Can I use ordinary spray paint instead of Tamiya's TS or PS lines?
On ABS it can work if the paint suits the plastic, but results vary. On polycarbonate it's a real risk — general lacquers can crack or craze the flexible clear plastic, which is why Tamiya sells PS as a separate, polycarbonate-safe formulation.
Does a heavier paint job risk failing the weight or size check?
Paint adds only a small amount of weight, and 90g is a floor for the whole car, not a ceiling on the body. The practical risk is heavy build-up eating back weight trimmed elsewhere; thin coats keep it a non-issue.
Do I need to prime a polycarbonate body before backspraying it?
No. PS Spray goes directly onto the clean, dust-free inside surface. Priming is an ABS-shell step for exterior colour coats, not part of the polycarbonate process.