Forum member Randydance suggested to me to take a new DE or SE blade and practice the TPT on that. This worked very well for me. YMMV. I don't know why it is suggested to moisten the thumb with the TPT or the thumb nail with the TNT but I suspect it is to lubricate the surface. Less chance of a false positive ? In the barber manual excerpt it specifically says not to use saliva. There may be corrosive properties in it that could damage the edge if it were not completely removed.