Is ペンキ塗り立て a common expression? If not, what will be a typical sign warning of wet paint?
Yes it's the most common expression on Japanese "wet paint" signs. You can buy stickers and tapes with ペンキ塗り立て.
Yes, it is- Allow me to explain. ～立て（たて）is a piece of grammar added to the stem of verbs to signify that they have been freshly or very recently done.
「習いたての単語を使う」→ to use a recently learned word
「焼きたてのパンを食べる」→to eat freshly baked bread
Therefore, the verb 塗る（ぬる） or to paint, being that it has ～立て attached, expresses the freshness of the paint job.