Making a weather resistant door

My house has an exterior door on the South side for a small closet that holds our gas water heater. The door is 24" by 48". My house is in Los Angeles so there is lots of sun, some rain and no snow.
When we looked at the house before we bought it, there was a plywood door that was falling apart. The termite people replaced the plywood door with a hollow core door.
The hollow core door soon started coming apart because of the rain getting it wet. I looked at Home Depot but did not find a door that size so I glued it back together with a construction adhesive in a caulking tube that was supposed to be waterproof.
The door is coming apart again so I want to make a new door out of exterior grade or marine grade plywood and coat it with several coats of spar varnish, marine grade epoxy or some other finish. I want something that will last at least 20 years.
Marine grade plywood is quite a bit more expensive than exterior grade. Would it be worth the extra cost in this application?
What would be the most cost effective finish for this application (assuming my time to be worth $10 per hour)? Thank you in advance for all replies. -- Whenever I hear or think of the song "Great green gobs of greasy grimey gopher guts" I imagine my cat saying; "That sounds REALLY, REALLY good. I'll have some of that!"
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Any exterior door material properly painted and sealed should last 50 years or more.
If you want or have to build something, I'd use the marine grade plywood and frame it with solid wood to help keep water from getting in between the laminations. Over time, that can cause problems. I'd also look at using MDO. A sheet of MDO looks like plywood and is the material used to make road signs. It has a smooth finish on both sides and can be painted.
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Daniel Prince wrote:

No. Coat it with epoxy, especially the edges. After a week or so, wash it well to remove the amine blush. It will still probably warp.
IMO, you'd be better off making a stout frame and covering it with well attached ply then epoxy.

Paint. Lasts best too. Varnish will require constant maintenance. If you varnish, it is pointless to use spar varnish.
--

dadiOH
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We have a south facing fiberglass door that looks good as new after 15 years.

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Do you know where I can get a fiberglass door that is 24 inches wide? -- I don't understand why they make gourmet cat foods. I have known many cats in my life and none of them were gourmets. They were all gourmands!
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alt.home.repair:

24" is a standard door size, but most places don't keep it in stock. Go to the customer service desk and order it. You'll wait 1-3 weeks.
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Steve B.
New Life Home Improvement
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