Some professional painters do work on other projects apart from house painting.
However not all professional painters work on just houses and not all professional painters work on other projects.
It depends on their skill and what they advertise they do.
However some professional painters may work on other projects if you ask them.
But for example a house painter would not likely paint a car and then you would have to get a car painter for the car.
But it all depends on who you hire.
Some painters do more than just painting houses to keep their work flow going and to keep earning money.