Bill has it—most commercial detergents have some sort of skin conditioner, or they have something to prevent water spots, or they just have a formulation that leaves something more or less innocuous on the surface. It only takes traces. Simple Green, Formula 409, and Windex all work well. All should be rinsed with water, followed by an alcohol (isopropyl or ethyl or denatured) wipe.
If you have to wash the model between coats, I suggest Woollite, followed by a distilled or deionized water rinse. If you are using enamels, this can be followed by an alcohol wipe. If using acrylics, do not use alcohol on the painted surface.
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Bill, nice to see you posting again!