I picked up a brand new, prehung exterior fibreglass door from the Habitat Re store this year...$175, yes brand new
. I couldn`t pass on this baby, right size and hinged on the proper side, still in a shipping brace .It was custom made without a threshold so before buying I contacted the local supplier who donated it to see if they can supply a threshold. They suggested I come to their factory showroom for stain info as there are recommended coatings.Haven`t been yet but I think this applies mainly to stain but I also work with a custom builder who told me to stop into his specialty paint shop as well for the ``proper`` stain . He just hung a set that must me 8`tall.
The only problem I have now is that once I do the install my future patio doors, almost adjacent will likely need to match so $$$$$$ for them
Might be a good idea to double check at a good paint store just in case there is a preferred undercoat.
FYI the fibreglass that I have seen locally looks beautiful and is said to resist warping from heat and moisture and provides better insulation according to the manufacturer