I’m with Racer - I would use sodium precarbonate or SH. I had a 15 year old fence I played around with that was in way worse condition with heavy mold/algae and rotting. I found precarb worked great except for the mold/algae where I would hit it with SH.
On this note, I have found that anything with sodium in it seems to encourage furring more than just plain water. Hydroxide seems to fur the worst with SH with a gradual increase in furring as the strength went up.
On wood, I think Hydroxide is only used when you have a stain or sealer to contend with.
