Varnish is not a finish that should be used in this situation. Maybe you can do it yourself. Depends on your skill level and what results you are looking for. From the description the door is stained and top coated. Can you fill the cracks with a stainable exterior filler, stain the door and seal it against the weather so it looks good? The door will need to be stripped of all the finish. Sanded, restained and top coated with a marine polyurethane(2-3coats)
Ron

Besides the minimum 3 coats of marine varnish(5 would be better), you have to maintain the door at least yearly with a light sanding and another coat of varnish. Doing that will keep the varnish from cracking & having to strip the entire finish. Once the cracks let in water the entire coating starts to come up. A west facing door is tough on any finish because of the intense heat.

I re-stained and re-varnished my front double doors last year. They face east, so they get morning sun, and the bottoms were in rough shape. I sanded off the old finish, used a wood moisturizer, filled some spots where the P/O's dog had scratched to be let in, re-stained with a gel stain, then applied 2 coats of marine-quality varnish with UV protection.

The results? Well, there are some flubs, particularly in recessed areas that were hard to sand. It takes longer than one thinks it will because of all the sanding and waiting between coats of varnish and/or stain. There are things I would do differently now that I understand what to do. From the street, though, it looks great. I am just glad that the bottom no longer looks like driftwood.

Have a beautiful front door, facing South In La. I stained the door then--Mistakenly put a water base varnish over it.
The varnish is turning milky. I was told I could wash the door with vinegar and then just revarnish with an oil base
varnish. Will this work???