On Friday, September 20, 2019 at 10:41:24 PM UTC-4, Oumati Asami wrote:
I have a staircase which is under direct sun light (through glass
window). After 20 years, the top coat is gone. Usually, when finishing
wood, one would applied a coat of sealer to seal pores on the surface of
wood. Would this sealer coat also be decomposed by sun light?

Find one rated for outdoor use, eg spar urethane. Is the coating gone
all over, or just where it's worn down from traffic? I'm assuming the
former, otherwise it's not UV related.