Many threads have referred to using auto primer paint for brick and concrete coloring. I have also seen threads suggesting that they have used it successfully directly on plastic without damaging the surface. In general, are auto primer paints safe on plastics or do the different manufacturers use different plastics, some safe to use it on, others not? I would hope that a manufacturer would use the same plastic formula on all their products, do you find this true?
I have tested various non latex paints on foam, but foam is plentiful. I’d hate to try it on a structure piece and find it incompatable. Do you test a paint by using part of a sprue or can you use the back of a wall and not effect the other side?
I know there are primers made especially for plastics, but the ones I have seen would need to have another color put over them to be a brick color. This could possibly cause loss of detail.
short answer – it “might” be. Check as you said – on a sprue.
Plastics are usually pretty tough so you shouldn’t have any trouble (as always, test first). Many of the model paints that you use are likely solvent-based (personally I use the Polly Scale acrylics… don’t give me a headache when I’m airbrushing (and cleanup is a whole lot easier to boot)).
Depending on the type of plastic (since you’re talking structure, I’ll assume styrene) – you can probably do just fine by spraying paint of the appropriate colour right on the parts that need painted without too much issue.
I use Testors primer in a spray can – white if the top coat will be light, gray if it will be dark. It goes on very thin (if you know how to spray paint, that is) and doesn’t hide detail. It seems to shrink a lot as it dries, because when first applied, there is some loss of detail, but the detail returns when the primer is dry. The key is to let it dry fully before you paint.
I keep a box of styrene scraps that I test new paint brands on before I use them on something valuable.
If I am going to be hand painting or brushing color on a building, I will prime the building with plastic model paints that are flat. Generally light gray.
If the building is concrete or brick, i will spray it with plastic model paint of the appropriate color.
Paints that are specifically made for models have a finer ground pigment. Because of that, they don’t fill in the details unless they are sprayed on very heavy. This is the reason that I don’t use auto primers or other cheaper spray can paints found at general hardware stores etc.