I have painted redgard on the shower walls and I am ready to tile. Which thinset mortar do I use over the redgard? I am using 12x24 tiles on 1 wall and 4x12 on the other 2. I am using ceramic tiles.
Some mortar/thinset is not suitable for porcelain. Porcelain should be specifically listed if your tiles are porcelain. Floor tiles could be porcelain. Wall tiles are usually ceramic. The labeling of your tile should reveal this information.
This refers to the substrate, as exposed on the back of the tile. (The finishing on the front does not matter.) Ceramic is lighter, more porous, and reddish. Porcelain is heavier, more dense, and often white/beige.
Many bags in the tile-aisle look the same, so read the long list of application/uses. The thinset for porcelain is more expensive, and can be used for ceramic. For any questionable backing or tile type I use thinset for porcelain.
Using modified thinset on a roll-on water membrane is fine.
Make sure you applied the membrane thick and with multiple coats. It is not applied thinly like a primer. Inspect it carefully, looking for tiny bare or thin spots. Let it dry and cure per instructions.
Find out the brand sold at the store where you plan on buying it. Call the manufacturer, they're the best source for a question like this. They'll tell you the best product for your application and also tell you what size trowel to use. 12x24 tiles definitely need the proper thinset. I would also recommend you use a "tile leveling system" for the 12x24 tiles. Hard to explain what they are but you can easily find a video. They are clips that keep the tiles level so you don't have a tile that sticks up higher than the others.
The answer depends somewhat on what type of tile is being used on the walls. If the tile is a common ceramic or porcelain, you need a modified ( just means that polymer bonding agents are added) thin set. If it is a stone tile a non modified is recommended. I would use a thin set for medium format tiles. It can be used with both size tiles. Good luck.