what kind of coat is always wet when you put it on
A coat of paint.
Why it works
The riddle uses “coat” in the sense of a layer, not a piece of clothing. A fresh coat of paint is wet when you put it on.
Other common answers
- Raincoat, if someone takes the riddle literally.
- Overcoat, as a playful but incorrect guess in some forum replies.
Quick note
This is a classic wordplay riddle, so the intended answer is a coat of paint.
TL;DR: The answer is a coat of paint.