Yes! You can paint UPVC doors, just be aware of the limitations of painting mineral paints on shiny plastic. Although you can paint them – it is hard to do a good job (generally painting on plastic), so please be aware of this before you start. You may find that spraying the door gives a better finish than a brush.

The ideal way of painting these doors in order to get a smooth surface would be with a spray gun after you remove the doors. This, however, is an extremely lengthy process and requires specialist tools, so it is still possible to use a brush. 

Clean the surface of the door  to get rid of any dust particles or grease that could compromise the attachment of paint on the surface. After cleaning the door, give it a key sand and brush off the dust before applying the primer. Using sandpaper will create a smooth surface for the primer to stick to. 
Use this universal primer to create a surface that will allow the paint to adhere. Using your paint brush apply the first coat of the primer and wait for it to dry. You might require an additional layer of primer depending on the condition of your upvc door. 

Before you start painting with this natural multi surface paint, use a good quality brush that won’t leave behind any brush hairs. You will have brush marks, you can’t avoid that if you are painting a upvc door but you can avoid brush hairs stuck in the paint. Once you are done painting, put boxes in front of the door or any other hurdle that will prevent kids or pets from leaving marks on the door. This paint has a quick drying time and will dry in a couple of hours, and will require at least one more coat. 

Try a small area first if you are worried about it!