Paintguy is right. Here's my story. Too late to help now but maybe someone else will benefit. I gel stained my Pella fiberglass entry door 6.5 years ago and it looked great! Used Minwax gel stain and their spar varnish, as directed by Pella. However, it did not stand up to weather at all. It chipped and had to be varnished again every 6 months. This Spring it chipped all the way through to the fiberglass and pulled stain with it! Believe me, I did everything properly. My door faces south east and has no roof. But a neighbor whose fiberglass door faces north had the same trouble - he used the same products as I. My neighbor got disgusted and stripped his down to fiberglass and restained it with Sikkens products 3 years ago. No trouble since. Sikkens makes a Door and Window stain that does not require varnish over top. Fortunately for me, just before I started stripping about 36 layers of varnish off my door, it delaminated! Pella brought me a new unfinished slab (under warranty) and I'm using Sikkens to stain it. I've been told the varnish is the trouble. It's like putting a sheet of glass against the door, trapping the heat. Plus it isn't flexible enough to expand and contract. If I put up a storm door to protect it, the problem would've been worse due to trapped heat. Sikkens is specifically made for fiberglass doors. I don't have any vested interests in the company but thought I'd try to help the next guy... Here is a link that might be useful: Sikkens Door and Window stain

Are you going to paint or stain them? We bought fir doors, and ours are stained. I think they look nicer than stained pine would have. Fir is also a denser more stable wood, but I'm not sure if/how that affects their long-term performance.

I've always used Regal Select from BM and it is my favourite paint. I've used other brands and I always come back to the BM. It has great coverage and goes on nicely. I've never sprayed it. But if you are thinking of it for your exterior....look into Maibec siding. They have many profiles and they have their own set of colours or you can have them custom tint to any major paint brand. We had ours tinted to BM colour and we are very very happy with the way it all turned out. That way it is prefinished, and no need to paint it or spray it onsite.

Thanks for the fast responses. TruStile is not in the budget but Steve's Doors is available through Reeb locally and they have good construction info available. I will eliminate the 1 3/4 hollow core option, I suppose it would make more sense to do 1 3/8 solid core if it becomes a crunch budget issue. Any thoughts on hollow core doors for closets?