[kokopedal]

Desaturating or greyscaleing a picture is not the way to convert a picture to B/W. the best way is to select a new adjustment layer, then click on channel mixer,then click on the bottom left where it says monochrome. Adjust the sliders to the desired effect. the general rule of thumb is that all three sliders should total up to 100, however this is just a general rule and most of us don't like to play by the rules. This way will allow you to bring out the black and white tones you desire. You can also use this method to convert to IR (Infra red) There are other tweeks involved but try this and play with it I think you will be happier with the effect. Also rember that if you use the history brush instead of a layer mask you probably will loose the ability to go back and make further changes.