I recently bought a bottle of Model Masters Clear Flat Acryl and I've had great success with it. You can use it as a top coat or I've added it into some of my paint mixes that used gloss paints that I did not want to have a glossy finish. You can to add it to the Model Masters gloss colors to "dull them down"

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Acrylics are water based, you shouldn't have any trouble with them reacting to the plastic, they need more surface prep to adhere. Give the figures a good hot water and soap scrubbing, and clean up the joint area. I've found out the hard way.
Enamels hate softer plastics, and don't even think about lacquer. Lacquer hates everything, including other paint.

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