I have mahagony stained furniture in my bathroom. I have a touch up stain pen in mahagony from homedepot for when my cats nails leave trails. It works great. All my pieces are old furniture in the bathroom. I sanded and restained them all with minwax and finished with two coats of marine varnish. Without cats, the finishes after two years would still be flawless but I do have a properly sized fan and my mirrors never fog even with long hot showers.

I would recommend a factory-applied catalyzed conversion varnish for cabinetry around water.

Honestly, I don't think it is much of a compliment to your cabinet finishing process to assume water or wear would ruin the finish under normal handling. A lot of the original cabinetry in our house has survived and wood can take quite a beating!

Like the PP, when we finished the woodwork in our bathroom we used marine grade varnish. 3 years here and no issues. I do love a complimenting stain pen in case accidents happen - I like the ones sold as a 'marker' and not a crayon.

This is really interesting, thanks all. I had no idea there was a stronger varnish option to avoid the typical wear I've seen with builder-grade cabinetry. What a great idea to use marine-grade varnish.