hi there
My son uses Methylated spirit which you can buy off the shelf – to remove acrylic paint from his model figures. Maybe the best one. You could brush it on and leave it on the surface.
Isopropanol alcohol (from Ebay) works good as well.
Sometimes you would need to soak the article for longer – up to an hour – to allow the chemicals to work. What may help you though – paint it on – and seal the model in a plastic bag to prevent the meths/isopropanol from evaporating.
Hope that helps you.
John