Originally Posted by
mark mcfarlane
Painting over the mineral wool will change both its acoustic absorption characteristics (frequency dependent, largest impact of paint will be in the mid-higher frequencies) and its fire resistance (one of the benefits of rock wool, which is actually made from basalt and chalk). I don't know what kinds of temps you will be encountering, or what frequencies you are trying to attenuate, or how 'light' a spray you can achieve while still bonding the fibers but this is something to consider.
If the temps are not very high, the traditional approach to encapsulating mineral fiber in an acoustic application is with a cloth material. Flame resistant material is used in commercial applications. If you can breath through the material it won't have a significant impact on acoustic absorption. If you can't breathe through the surface coat, then higher frequencies will not be absorbed, they will be reflected.
A final note, Roxul is available in different densities which also effect its frequency absorption characteristics.
Food for thought.