Any good outdoor paint will work as well as anything can. Fence paint, barn paint, whatever. I would not spend money on quality house paint for that job.
Or oil it. Linseed, or really, anything. Waste motor oil, transmission fluid, hydraulic fluid, etc., would work, but that would not be kind to the environment.
OR, you could accelerate the rotting. Drill as many holes as big as you can - at least half inch, and as deep as you can. Get some cheap corn syrup or pancake syrup. Generic. Cheap. Mix it half and half with warm water, stir in a bit of yeast (like for making bread), and soak that thing.
The ants will love it, the fungi will love it, and the stump (or most of it) will be punk in a year. Might take longer with some species, and you might need to clean away the punk and do it twice.
I recently tried that on a 36" sycamore stump. Worked great. It was going to need a second dose, but I had to rent a backhoe (John Deere 310) for another project, so I spent 15 minutes ripping out what was left underground.
BTW, that backhoe cost me a grand for the weekend, and I got everything done that I needed to get done, and had time to spare. I could easily have taken out ten stumps that size over the weekend.
(It was fun,, too.)
