Hi Nancy...
Yes, you can paint a metal or fiberglass door to look like wood! Yeah. And no... you don't need a graining tool. I have used a graining tool, but I usually don't like the results.
I would prefer that the door have a subtle grain... I try not to call attention to the fact that it's be "grained".
I have painted and faux wood glazed many doors of the fiberglass or steel type. They have very smooth finish with raised panels but it seems that paint would never stick to the smooth finish.
You must prime! Primer is available for exterior use and it's usually an oil based product. You can't sand the door to give it stickability, so you need to apply a good primer coat to give the paint and glaze something to stick too.
Pick a day that is temperate in climate.
* The primer will have time to cure and the weather must be neither too cold nor too hot.
* The door should not be shut for about 12 hours, or you will risk the primer sticking to the door trim instead of the door.
Zinsser brand makes an excellent primer for just this purpose. Although they state that the primer will be ready for paint within 2 hours, I have not found this to be true on doors that will be closed against a door jam.
If you are unsure or need more training… this video will be a huge help!
Faux Wood Graining DoorsHave Fun Painting!
Debra Conrad
THAT Painter Lady
Free Insiders Guide to Faux Painting - Save Your Time, Your Money and Your Ego - Get These 7 Faux Painting Tips Now!
http://www.thatpainterlady.com/