You may not be doing anything wrong. You're just trying to do something that won't work! Automotive paints are not intended to be applied on polystyrene plastic. While there are some formulas that will work, most will not. Also, there are many variations of polystyrene plastic. Most likely the chemical formula of the clear coat you're using is incompatible with the styrene that this model is made of. Always remember to TEST FIRST before painting a model with anything other than paint made for polystyrene plastic.