How much solid deodorant should I use and do I risk white marks on my clothes ?
When you apply the deodorant to your underarms, it should be well moistened and glide over the skin, just like a bar of soap between wet hands. It leaves a fine, transparent film on the skin, which dries quickly, so there isn't any risk of it leaving white marks on your clothes. For maximum effectiveness, I recommend going over the underarms well, both lengthways and widthways. A fine layer that covers a large area will be more effective than a thick layer over a smaller area.