This method has its limitations; however I don’t think it can be beaten for doing areas of high wear such as cupolas and the like. Take a ripped piece of sponge, packing material and kitchen scourers are ideal. Dab the sponge into the paint and then dab off the excess, then lightly dab the sponge onto the part. Keep things light and slowly build up the effect. If you go too far don’t worry, just re-spray the effected area and start again. That’s all there is to it.
As ever it’s much easier to show you so here goes – good luck!