I have a vintage door I am wanting to paint to use as a headboard
Most just paint with a white semi-gloss paint or use a whitewash on a unpainted door for a rustic look. Then hang on the wall with a French cleat, either a purchased one or a home-made wooden one. https://www.youtube.com/watch?v=DmLQkRDOgOE
Make sure you test for lead first if it's a very old door, just in case, and take the necessary precautions if you need to sand off or powerwash off any old or chipping paint. If there is old paint remaining chalk paint is really quick and easy way to paint over it with minimal prep. Due to the weight, hanging with a french cleat is a stable way to mount it behind a bed. You can even had some trim around the edges if you want to bulk it up.
Unicorn Spit. It comes in many colors and is easy.