MAG-PAINT has metal in it to make the magnet stick.  Recommended: apply 2-3 coats over each board tops surface.  Use a roller to paint it on. Let each coat dry 30 minutes before applying the next coat of mag paint.
 
MAG-PAINT: Used to make a magnet cornhole wrap cover stick to your smooth surface cornhole boards. 
 
Multiple Uses:
Can be used for cornhole boards, walls, or projects on a smooth surface.  It can be painted over with standard paint but requires more coats 3-4 for best results.
 
2nd Option: Can be purchased on Amazon, 1 quart Size, Click Here - there is enough to cover a set of board tops