There’s a weather window tomorrow, need to get the 3 week old render painted before winter kick is in. Do I use a 50/50 mix as recommend by the builder or a 10-15% mix by the guy who sold me the paint? Bit nervous as I’ve never done outside paint!
Will it fall off?

Is that 10-15 water/90-85 paint?

Personally, I’d go 50/50 first coat. Then 15/85 second.

mixing what?

*Apologies or he slight hijack* Our builders are nearing completion of our extension. I’ve asked for smooth coat render with the cement mix coloured white so I don’t need to paint it. Has anyone experience of this? Also is a simple sand cement render best for a new timber and block build.. Cheers’

What sort of paint? The pail might have guidance on or if it is a mainstream brand then they’ll have tech notes on a website somewhere. Dulux do and they may have something relevant to you anyway.

50:50 will leave it piss thin and you’ll definitely need multiple coats. 10-20 is normally OK and what you would use for fresh plaster. If you thin it it will sag/drip as the thixotropy will be all wrong so go easy on the thickness you are putting on. Personally I’d go off the guy who sold it not the builder unless you can get hold of the chemist who formulated it?

You’ve got two choices:

Follow your builder’s instructions (who probably is repeating some other builder’s instructions)

or

Go to your professional paint shop and buy a can of Johnstones AND FOLLOW THE INSTRUCTIONS WRITTEN ON THE BACK OF THE TIN

I paid about 50 quid for 5 litre can, if I remember

I was going to say maybe not quite 50/50 but I’d certainly dilute the first coat considerably. It’s just a first coat that you want to soak in and firmly adhere to the render, the next coat (no or little water) will be easier to apply and will look better.

I am only a DIYer who hates painting though.

Go to your professional paint shop and buy a can of Johnstones any good quality exterior paint, AND FOLLOW THE INSTRUCTIONS WRITTEN ON THE BACK OF THE TIN

^This, not some builder who is remembering what someone told him 10 years ago about Wickes own paint etc….