Also along those lines, did you notice that whenever they put those sticky sensors on him to get heath readings, they are sticking to his head. Kinda hard to do considering we can see that he always has hair.
When you put latex mask on your head, if it's heavy enough, it can mat down your hair. It wouldn't be the same as light-weight blood that was thrown on him.
The thing with his hair is that when they made the movie Kevin Bacon always had to wear that green suit and that had completely cover every part of his body including his hair, making him look bald whenever they did things when he was invisible! The thing with the blood is that it was something different as to making the mask for him to wear and the water was made differently too they actually went back and designed it to look real like that with the computers! if you have seen the DVD version you will know why if you go through all the deleted scenes and production of this movie!