Projector screen paint - make/paint your own screen

Hi Folks

Looked at a couple of AV threads from other sites mainly US based and wondered if anyone has played with making there own high quality screen. There seems to be a few sellers of paints but checking them against AV forums some can be complete hoaxs for example this one

https://www.crystal-edge-technology-screens.com/

if you go by what these dudes are saying

http://www.avsforum.com/forum/110-diy-screen-section/2500930-crystal-edge-technology.html

http://www.simpit.co.nz/ have some amazing stuff and loved the screen Thomas(really nice guy and am considering his product price for a 120inch is $1500 free shipping nz) is selling but was thinking as we are building a house has anyone had a crack at mixing paints and come up with a formula or paints that can be brought and mixed in NZ. With a goal of possible much larger screen

So questions

1 anyone had a go at making/painting a screen

2 what is the down sides/learnings from this?

3 what makes a $2000 screen better than say a well rendered wall with the right mix of reflective black paint. - assuming labour/time is not a barrier as am keen to have a crack :)

4 Any links to genuine paint sellers with reviews or people who have used a product?

5 should all stars align what would be the largest possible picture in 4k this projector would be able to make in a darken media room Optoma UHD65 4K Home Theatre Projector