Can you use poster paint as car paint?

No, you absolutely should not use poster paint as car paint. Poster paint is not designed for the harsh conditions a car's exterior endures. It lacks the durability, UV resistance, and flexibility needed to withstand sun exposure, rain, washing, and impacts. It will likely crack, peel, and fade very quickly. Using poster paint on a car will damage the car's finish and will not provide any lasting protection. Use automotive-grade paint specifically designed for cars.