My method for testing out a paint scheme is to use a shield or a bit of plasticard. Please note, the link is for illustration only, plasticard or polystyrene sheets are widely available at many hobby shops, craft stores, online sources and costs vary widely. In stores, they go by brand names like the Galeforce 9, Army Painter, Plastistruct, and Evergreen, but they're all the same. Its a sheet that's mainly used for kitbashing, scratch building, etc, but can be used for other mundane purposes.

What I do is make a small, manageable size by using a metal ruler and a hobby knife to score a line across the sheet, doing it several times. I can then snap that sheet across that scored surface to get a smaller sheet. I'll do that until I get a 40mm or whatever, sized square/rectangle, whatever, and just spray-prime that sheet. I then use that to test paint schemes.

If I dont like it, I can re-prime over it, and start again. Its cheap and you can always use the plastic card for other things.