It'll still take water in what do you think happens when you skim sand cement if it didn't absorb moisture it'll all come off in sheets
Freed why can't you mix lime and waterproofer
Thats the problem with putting it in the top, when it gets in it will freeze and blow the top coat off, Its not completely water proof, your top coat is supposed to absorbe water then it just dries out but will not get past the scratch.My worry with putting no waterproofer in the top coat is that if it isn't painted, rain/moisture could be drawn into it, freeze, expand and damage the render.
Thats the problem with putting it in the top, when it gets in it will freeze and blow the top coat off, Its not completely water proof, your top coat is supposed to absorbe water then it just dries out but will not get past the scratch.
Hydrated lime is a by product of slaked lime it doesn't help it breath it lightens the mix like an extra shovel of sand and splits the particles like a good feb mix I've used it with waterproofer and there's no reason not too Ive had too because of thickness Sand cement is brittle including hydrated lime don't get carried away thing hydrated lime is the queen bee and will stop everything cracking and allow it to breathe if you want that you need to look at hydraulic lime
This will happen whether waterproofer is in the mix or not. At least if there is waterproofer in the top coat it won't get as saturated. Lime in the top coat will only draw more water into the top coat increasing the amount it retains increasing the chance of damage to the render, especially in high weather exposure areas. modern paints e.g Weather shield are breathable.
Modern bagged renders have waterproofing additives in them and are not left out of coats.