What is the past participle for voir?

The nominal form is the infinitive used as a noun, so it is always the same as the infinitive. In that case, "voir" (to see).

Example: "Voir rouge est un défaut" (literally "to see red", meaning "to be angry", "is a fault")

"Voir" here is used as a noun (nominal) and is the subject of the verb "est" (is).

It is not a common verb to use in nominal form.