How do you repaint a fiberglass door? What is the prep?

Matters not that it is fiberglass. Painting is always the same. Clean, sand, clean, prime, paint.

interior or exterior?

Yes. Clean with TSP, sand, clean, prime then paint with a good quality exterior paint.

Use a stiff wire brush or a paint scraping crevice tool to remove the finish from any recessed scroll work. Sand the entire door with coarse, medium, and fine sandpaper. Completely wipe down the door with a damp cloth to remove any dust, and allow the door to completely dry before continuing.

clean sand and prime !!!

Use Gel stain. There are many colors to choose from at lowes. Clean door and apply.

Appears to be an exterior door. I'd lightly sand it with 220 grit sandpaper, clean with hot water & TSP (wear gloves). Let dry. Apply several coats of Kilz2 allowing for proper drying time between coats. Apply several coats of exterior paint in the sheen of your choice, also allowing for proper drying time between coats. Voila! new door. Best of luck.

When I replaced all of the windows in my house, I also had new fiber glass doors and new screen doors put on. The front and back doors are white, and the sales person I had told me that if I ever wanted to paint the doors, to go to a Paint Store, like Sherwin-Williams. Since all they sell is paint, they are the most knowledgeable.

Fiberglass doors are often preferred for their mold and rust resistance. Unlike wood, they will not deteriorate or swell. While fiberglass doors can be painted, there is a bit more effort, involved to ensure the finish lasts.

Painting fiberglass doors require some planning. As there are different types of fiberglass, there are different types of paint and knowing what to use for a proper bond is especially important. Types of fiberglass include epoxy resin and polyester-based resin. They require epoxy-based paint and polyester-based paint, respectively.

This link is to the DIY website, and it describes resin types, primers & paints to use. If you do paint your doors, please take pictures when they are done, and post them here. I and others with fiber glass doors would really appreciate it. Good luck !!! :-)

https://www.doityourself.com/stry/types-of-paint-to-use-for-fiberglass-doors