Hairline Cracks in a Solid Wood Door

We have a beautiful solid wood front door. The problem is that it has hairline cracks in it. How would you address sealing up cracks like that?

It's currently finished with Australian Timber Oil and I'd like to not paint it or add another kind of finish over the top. Solid wood doors are very expensive, so I'd like to rescue it rather than replace it if possible.

I'm no woodworking expert, so here's my 2 cents' while we wait for the gurus.

I wonder, are your hairline cracks all vertical and evenly spaced? Solid wood doors are generally laminated, meaning they are solid wood boards glued together. Often a veneer is added, for a consistent look and to protect the core.

From what I've seen, laminates act as a solid piece of wood unless variations in humidity and temperature, including direct sunlight, start to add stresses as the boards and glue expand/contract at different rates over time.

Personally, I think the best look is still the medieval solid wood door with wrought iron strapping holding everything together. Not sure that helps your particular situation though.

Disclaimer: I am by no means an expert on wood doors.

That being said, I think there are basically to a approaches:
- accept it is a natural material that will age
- prevent aging by any means and encapsulate it in plastic (clear polyurethane finish maybe?)

Since you already coated it with oil, that would have to be sanded off first for any plastics to work.
I would probably just apply another coat of oil and fill the cracks with wax.

I have had a lot of experience with wooden doors.
Some questions;
- how old is the door itself as a door?
- Does it get direct sun on it?
- Is it in a hot environment?
-Does it get water saturated?

If that were my door I would treat it periodically with the oil.

When was the last time the door had an oil treatment?

Lots of questions so I'll do my best to answer. I appreciate the wealth of information

The door is probably as old as the house which would put it at about 30 years old.

It gets a lot of direct, afternoon, Texas sun. It's over 100 degrees in the shade today. When we moved in, the door had gotten to the point where the old polyurethane was flaking off so I stripped it down to the wood and refinished it with timber oil a couple of years ago. I need to do a light sanding and refinish again so I'm trying to figure out how to remedy the crack situation.

The door is under an overhang, so it doesn't get water-saturated unless it's raining sideways, which can happen.

The cracks are vertical, but not evenly spaced. The door is made up of panels of solid wood so no veneer. The cracks appear in the wood itself and not (just) in the spaces between the panels. I got a picture of the door itself and a closeup of some of the cracks. The cracks are hard to see, but you can see daylight through them when the sun is directly on the door.

Someone mentioned filling with wax before refinishing. Would that be beeswax or another more specific product? Is there a high melting point wax that could hold up to the sun?

I imagine the splits are in the thinner timber which is simply being dried and cooked by that heat from the sun.
If you could have a wall, even an arbour to provide shade on the door when that sun you spoke of is around that would help.
A wax of the right colour would help but it will need regular [ 6 monthly ] applications of oil to prevent more damage and keep it looking smart.
Some oils have a little bit of terebine to speed up drying and a bit of re4sin to give a bit of toughness.
Watco make a sauitable exterior oil in North America.

I agree that shading the door is the first step. There are not many materials that can survive those temperature swings every day and keep their shape.
And the UV radiation will slowly (or not so slowly) eat away any oil or wax coating.