I see some say it's a no no to use on shower walls w/1/2" hardibacker. Is this true and why?

If you did any reading at all you should know why. Mastic in wet areas will not work very well, especially the lower portions. If the mastic gets wet, it'll re-emulsify as it was in the can. Also since it's an organic adhesive, it'll feed mold. I only recommend mastic for dry areas such as a backsplash or an accent wall in your dinning room.

Take it back and get the right stuff. Thin set mortar come ONLY in powder form in a bag. There a several types in many qualities.

A few more details might save you grief later too. Like, how did you build the floor and curb? Any plans for a vapor barrier or waterproofing? Did you read any of the project how-to articles located on the main page of this forum? http://www.ontariotile.com/ Showers are not easy and it's easy to miss a step or three if you don't know.

Jaz

You know the only thing that Custom recommends against using Acrylpro on with regard to shower installations is shower floors? The data sheet says it's good for intermittent water exposure, and they never mention shower walls.

I still wouldn't do it, no matter what they say. Mastic is mastic is mastic. A top of the line thinset is still much cheaper, and you don't have to worry about it re-emulsifying later or growing mold on it.

Thanks for the responses, but it does say right on the bucket suitable for wet areas such as tub surrounds and shower walls.

It may say that on the bucket, but the Technical Data sheet also says " do not use in critical water exposure areas" and " provides maximum resistance to intermittent water exposure"

You can interpret those statements however you want.

My professional opinion is to use cement based thinsets anywhere there's water exposure.
As Kman stated, mastics can re-emulsify if there is enough water penetration. We tell you this because its a known fact.

Why risk hundreds or thousands of dollars of materials and labour just to be able to dip your trowel in a bucket of mastic.