Carrera 4S. Clio 172. M5
I have used this stuff for its original purpose for years, ie painting models.
It crossed my mind it might be useful if someone was painting small items on a car.
Its called Humbrol Maskol. A sort of liquid masking tape !
Its a rubber solution that can be applied to surfaces to prevent them being painted. When the paint has dried the Maskol can be simply peeled off.
You apply it with a brush and it dries quickly, so if you get it wrong the first time you simply peel it off and correct your mistake before you go near it with a paint brush or air brush.
You can buy it in Model shops.
FWIW