I have used chalk many times, and I believe it is similar to pastels. It's all just shaved dust of a pigment. If you plan to use chalk, just shave it off of the rod with a knife or razor blade, and collect the fine dust produced. I did this to fill the mortar lines in a scale building I was making. With a little water added to the fine chalk dust it simulated mortar perfectly. I just wiped the excess away with a wet finger, and it looked great. I flat coated afterwards. I don't know if I would use this for weathering a mecha kit, but it never hurts to try. Again I don't know of how different it is than pastels from tamiya, but essentially I believe it should all be just pastel chalk dust compacted in a little plastic container. Don't quote me as I'm no expert on manufacturing of pastels, but from my experience I believe this to be the case. I apologize for posting a picture on your topic, but this is to show what pastel chalk looks like when using this method. To avoid any unpleasant itchiness, I would just flat clear coat it to seal the pigment.
