If it's acrylic white spirit or acetone won't work, if it's enamel white spirit will do the job fine, better again, enamel thinner, but be warned, it stinks, don't use it in the house, it is one of the most common causes of model induced divorce.
Actually if you have a petrol lawnmower, use a bit of petrol out of that, will also work on enamel, if it is acrylic, just use water,