Yes, you can apply acrylic paint over spray paint. Acrylic paints adhere well to spray-based paints. If the existing spray paint was water-based, you can easily apply acrylic paint over it. But if the spray paint is oil-based, you must first remove the spray paint.
Acrylic paint is the best paint to use over spray paint. Acrylic paint has polyacrylates that adhere well to spray paint, as long as the spray paint is primed. Yes, you can paint over spray paint as long as it's completely dry, not just to the touch, but it's actually dry and cured. You have to sand the finish lightly with 100 grit sandpaper to provide something that the new paint can adhere to.
If you sand too soon after spray-painting, you'll create rubbery dirt. After sanding, wipe the surface thoroughly with a damp cloth and let it dry. You can then paint over the spray-painted surface with any type of paint. Artists have been known to use acrylic paint on their spray-painted base for their works of art.
Aerosol paints are practical, since they can cover a large surface area, in a relatively short time. If the spray-painted surface is smooth and even, it will allow the acrylic paint to adhere without problem. It will work well, if you wait long enough. You shouldn't apply water-based spray paint to oil-based spray paint (or vice versa) because the paint won't stick.
I bought a can of primer, next time I'll start with that, but it doesn't help me with the ones I've already painted. To remedy the situation, wait until the paint is very dry and then gently sand it with 100-grit sandpaper, followed by 220 grit. The first time I cleaned the surface and painted, I waited 3 days and then noticed that when I ran my fingernail over the surface, the paint was scratched. When creating art, it's important to know which paintings can be combined; otherwise, you run the risk of having your entire work of art ruined.
Use an aerosol primer from the Krylon Fusion line or the Rustoleum Plastic paint line, let it dry and then spray onto Fusion spray paint or apply a brush over acrylic craft paint. When using any brand of chalk paint that comes in an aerosol can, follow the same spray techniques you would use with a normal can of spray paint. If this is what you do, then it could be that you are spraying too much and only need to use lighter layers. If you paint acrylic on an oil base, the most common problem you'll see is that it lifts, cracks, or peels off.
Clean it: Print and then paint or use a spray paint made for plastic if you are going to paint with a basic color. I recently painted 3 different antique furniture with Annie Sloan Old White and I never worried about tannin bleeding, although each piece is made of pine and is handmade, because all 3 had been previously painted white. Although I was tempted to give up completely, I decided to scrape all the paint off the glass and remove the worst wrinkled stains from the shells with sandpaper.