Is fasting on Fridays, other than in Ramadan, considered incorrect?

What is discouraged in voluntary acts of worship, including prayer and voluntary fasting, is to single out Fridays for that purpose. Consequently, it is discouraged to perform a voluntary fast on a Friday, unless you fast either the Thursday preceding it or the Saturday following it.
There is no harm if there is a good reason for singling out Fridays for fasting. For instance, a man may not fast during Ramadan because he may have fallen ill. Or, his job may be strenuous, making it difficult for him to fast and his day off is Friday. In this case, it is perfectly acceptable for him to fast on a Friday to compensate for his missed fast-day.
And God the Almighty knows best.