That may be the wrong direction. I had the same prob when I lived in Florida.

Oscar suggested further diluting the ferric chloride, to 10% or even 7.5%, weaker as ambient temp goes up (reactions double with every ~ 10 deg C increase). Both worked well for me. Within the range mentioned, rate -- not effectiveness -- is the effect that changes.

Oscar worked in a relatively cool basement shop, a point to remember when using his excellent info.

Fred