Help I live in south miami-dade county FL. My back door is irregular therefore I have to use a 36x 80 blank and cut it off . How do I properly seal and finish this door ? This is the third door in 10 years HELP!

I am old fashion I like to use a regular stain and not that combined stain/ sealer it gets sticky and I don't like to cut it down with spirits The best sealer like to use is a spar urethane .To prep sand the door down real smooth and wipe down. To stain I like to use a rag or old sock to stain with and no brush (stain Only)so I can control the excess stain and not darken the door too much. ( I like to see my wood grain) I hate a brush on or sponge on. After this let it dry and NOT out in the sun door will warp badly. Now the stain will raise the grain. After completely dry lightly "hand" sand down the raised grain enough to smooth it real good but not too much to get past the stain. Wipe off good. Now before you seal it get some tack rags from the paint supplier and tack the door to get all of the wood fibers an dust off( real important).
Apply your sealer and coat the door with a good brush and smooth out good but don't go to heavy. ( cut this down with spirits if to thick and hard to brush out but not too much dulls the gloss finish) Lightly sand this coat and use the sticky tack cloth to clean it. The last coat is to go on. Here depending on the temp. You might have to thin a little for the sealer to go on smooth and a full wet slick look. Don't go to heavy. If this finial coat suits you good, if not do another finish coat waiting 24 hrs. Please don't forget to do the top and bottom of the door. These areas are forgotten too much and this is how a door swells because the bottom ot top edges are not sealed. Iam old fashion. I like just a stain no sealer in it. And for finish I like an exterior spar urethane finish semi gloss or high gloss. It is also good for salt areas and on boats. NOTE" I like to pour my sealer in a small container so I don't contaminates the can if I pick up any dust The tacking missed. Clean brush real good between sealing coats ( Dust again)

21boat did a great job. All I can add is I add Floetrol to my final coats to help the top coat to flash and get very smooth. Floetrol is good for latex and enamls so maybe not Spar. Floetrol Paint Additive (http://www.flood.com/Flood/CustomerSupport/FAQ/DIY/Floetrol+FAQ.htm)