Matthew Palmer's Watercolour Magic Mask
check it out at this link: https://watercolour.tv/shop/blue-mask-masking-fluid-60ml/
...Or as a set, with 2 brushes here:
https://watercolour.tv/shop/new-matthew-palmers-watercolour-magic-mask-20ml-masking-brush-set-free-how-to-use-video/
Join this channel to get access to perks:
https://www.youtube.com/channel/UCY3BpPdOOWgKJr4sSC0MrkA/join