For a long time now, I've been using art store paints (Guache) to do custom repainting or modifying of my toys.

But since it wasn't intended for this purpose, I had to modify the procedure a lot.
The first stage involves putting down a base coat of whiteout, since the guache won't bond to plastic directly.
Then once the paint dries, I have to add a second coat of clear fixative or it starts flaking off, taking the whiteout base layer with it.

Does anyone know if there's a paint that can be applied directly to toys without needing a base layer or a fixative afterwards?
It would have to me something I can handpaint with a fine art brush, since sometimes the details are quite small.

For example, my next project is to add yellow headcrests to Titan Returns Hot Rod, simply because this detail was missed.