You cannot adhere an adhered veneer to drywall, painted or not. You have 3 choices:
Screw 1/4" hardie backer to the drywall, hitting the studs.
Remove the drywall and screw 1/2" hardi backer to the studs.
Lath over the drywall (or bare studs) and lay down a scratch coat of stucco.
No matter which one you choose, use a polymer modified thinset to attach the veneer.
In some cases, you can adhere thin brick to drywall, but never over painted sheetrock. For thin brick on drywall, a specialized base-coat material is applied and then the brick faces are thinset to it. Suitable for small areas like backsplashes, but not for a fireplace or wall.
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