When I built the loco I tried to give the silencer an authentic rusty look using acrylic paint of the appropriate shade, but it was not very convincing.
The hatch, to which the silencer is fixed, is detachable from the loco body, making application of the chemical treatment much easier. While I was at it, I straightened the two exhaust pipes by applying some gentle heat to the right angle bends, they had bent outwards a little.
I am quite pleased with the result.