You are correct that it was rust that broke your OE springs. Those springs however were also faulty which was the combo that led to the failure.
However as stated, springs already come painted. If you paint them again, the paint only going to crack when the springs are used as that paint isn't made to flex with the springs.
Don't worry about it. Take a look at older BMWs such as e36's and e30's. The rear springs are often completely rusted on the exterior but the springs themselves are still fine.
'85 E30 325e
'90 E30 325i
'02 E46 325it
'03 E46 320i
'96 Volvo 850R