most of the fiberglass doors i stain are therma-tru doors,,,,,and they have a kit that i use.
it contains everything you need to stain the door.
they can come out flawless if you pay attention to detail.
i dont follow the directions in this kit,,,ive just perfected my own techinque.
ill shoot eh door with my hvlp, and tip it off with a wide chip brush. you can lay on enough material, and tip it off just right, and they look flawless.
for mainenance coats, i use a waterborne uv protective finish,,,,,and this should be done every year or two on a sunny side. if not stripping them gets COSTLY,,,so inform your customers.