Can acrylic paint be used on metal indoors?

You should be wary of using acrylic paint outdoors because it is not water resistant. If it rains, it will wash off or be destroyed by extended sunlight. If you use a lacquer to cover the paint, it may last longer, but oil paints are much more preferable. They can last 4-5 years and do not wash off in the rain or bake off in the sun.