Never used it, but I have occasionally used something similar MILLEFOLIUM MAGIC SPRAY
... It is ~$37 with S&H for a 4oz can. ProThik looks like a better deal if it does as good a job or better. I have had this can of MMS for about a year (tells you how little I use it) and it is about empty, so ProThik might just replace it.
I dont think it hurts the hair, but I would make it the last thing on (or next to the last if you set it with hairspray) if possible. Are you not wanting to wash it out for night topicals?