Yup, thats exactly what happens on your first attempts. and it probably will cost alot more. if youd like to get better, ya just have to practice like mad for years. find your comfort level with it, and find out what your struggling with exactly. Most importantly you have to have to have extreme patience! you cant just slap on paint and expect it to be any good. Go about it with care and understanding
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If you completely mess up.. then what you want to do is get some 90% alcohal or acetone, that will rip the acrylic paint that you just put down right off. Like an eraser. And try priming it a bit, with a good base coat of light grey or white. You can either do it with a brush and very water dulluted paint (multiple coats) or with spray paint. Spray paint is dangerous sometimes because you have to get the right kind of brand and be weary of what plastic you use it on. Spray paint is great for hard plastic, but not so much for soft.