A girl covered in water and then flour is pretty hot, but models don't do it because flour is a NIGHTMARE to get it out of your hair. That's what I recall reading, anyway. Is that true?

Is there any substance that gives the same look but is EASY to wash out?

Maybe white cake mix? Or what we like to do is cover my hair in conditioner first then use the flour water slime goo mix anyway with minimum flour balls.

Don't do it. It's a disaster and not at all worth any benefit you might see.

Douse her in oil first instead.

Yes, white cake mix.

use gluten free flour. it's literally just the gluten that sticks in hair - you wash away the starch and the gluten stays. I have tested this in sessions making my own gloop and cake batter with rice flour

https://www.youtube.com/watch?v=zDEcvSc2UKA

So between the two answers - white cake mix and gluten free flour- which is easier to deal with?

Gluten free flour is your best option. It's little more expensive than regular flour but easier to work with and clean-up.