SHOWER BASE: Redgard + tile

Hi folks,

I'm about to tile a shower base. The base consists of a Schluter/KERDI styrofoam tray and drain system. No Kerdi membrane. On top of the styro tray there is a skim of mortar and then 3 coats of Redgard.
QUESTION: Should a skim coat of mortar get applied on top of the Redgard before applying mortar & tiles with a notched trowel? Or, could I go directly to applying mortar with a notched trowel?

Right now I'm thinking to protect the Redgard from getting punctured by a notched trowel by first applying a skim coat.
Thanks

IMHO, none of those. The foam tray is part of a system, and not using it as designed will get you no warranty from anyone. TO obtain the desired compressive strength, the Kerdi fabric provides a HUGE increase in strength once things set up. You'll get none of that with a liquid membrane on top. Thinset sticks fine to cured RedGard, so no, you do not need to skim coat it with anything, wait, then add more thinset and tile. But, again, this is, IMHO, asking for trouble with nobody to back you up if you have a problem. Wedi makes similar tray that is not required to have a membrane on it (in reality, the tray is waterproof except for the seams), but not as much in vapor migration, and with the WEdi tray, if you're using smaller tile, they require you use an epoxy grout to make the tile hold together. Schluter's tray with Kerdi installed automatically ties those tile together, so normal grout works with any suitable tile (suitable in that you can get it to lay flat when installing on the sloped tray). Because what you are doing is not according to either products' instructions, it hasn't been tested, nor will you get either company to back you up if there's a problem.

Thanks for the advice, Jim.