Use any of the waterproofing sealants, such as Waterlox, Thompsons, etc. Or a couple coats of an exterior grade clear varnish (top and bottom of the piece)

About every year you will have to check it and maybe do a light scuff-sanding and a couple coats of finish on the top. thresholds get beat up quickly.

Caulk the edges so water doesn't get under it when you hose off the area (I'd start sweeping that area).

Varnish.

'Rock Hard' table top varnish (Behlen brad) does VERY well.

Around $30 a quart, but that should be more than enough.

Is it too cold now in the north to work with a threshold? We have some old ones that need to be sanded and sealed in our 1920-something house in Ontario.

I was under the impression that it was an interior threshold (also called a saddle).
Casey

"Is it too cold now in the north to work with a threshold? "

Not if you do the work inside before final installation.

You might have a caulk issue if there is not a storm door.

Go to a high end boat yard and ask for spar varnish. If you go to the right place they might have some left from a job.We always open new for every job even if we finished old job yesterday. They pay 59 dollars an hour for our refinish time and no one wants us to start with an old can or brush. Also ask for a small(3oz)tube of 4200 in tan to calk with it is the best but expensive. One of the perks where I work is 1,000,000 dollar yacht finishes on an old piece of furnature. I also have a piece of farm equipment primed with underwater metal primer and finished with Allgrip.

I have an oak threshold between a new living room addition of hardwood flooring and an older hardwood flooring in my DR. I just finished my threshold with the same Minwax poly that I finished the new wood floor in. Looks great!

On an exterior door I would use spar varnish and make sure you use a sill pan under it. Caulk and Minwax finishes will not hold up well.