**Your concern is justified: the base plaster’s compatibility with AAC matters, and a skim coat cannot reliably compensate for a base that cracks or loses its bond.** If you have yet to apply the plaster, I would choose a base render explicitly approved for your AAC blocks.

AAC creates several challenges for plaster:

- **High suction:** the blocks can draw water out of fresh plaster, affecting curing and adhesion. Surface preparation and suction control matter.
- **Shrinkage:** a cement-rich render can shrink enough to crack or separate from the AAC. Hebel’s technical manual specifically warns against strong, cement-rich mixes for this reason.
- **Movement:** joints and transitions between AAC and concrete can concentrate stress. These need appropriate detailing.

[Hebel’s technical guidance](https://hebel.com.au/wp-content/uploads/2016/11/Hebel-Tech-Man-Pt5-Fixings-Finishes-Access-2.pdf) discusses shrinkage and separation; its [application guidance](https://hebel.com.au/wp-content/uploads/2023/High-Rise-Facades-Design-and-Installation-Guide_HELIT015.pdf) includes suction control in hot weather.

Polymer modification can improve adhesion and toughness and reduce cracking, depending on the formulation. A suitable AAC render also needs compatible strength, controlled shrinkage and, outdoors, appropriate water resistance and vapour permeability. **“Polymer-modified” and “pre-mixed” alone do not establish suitability.** For example, [Hebel’s PowerBlock guide](https://hebel.com.au/wp-content/uploads/downloads/Hebel-PowerBlock-Design-and-Installation-Guide_HELIT043.pdf) expressly excludes general-purpose sand/cement renders, including bagged products, for those blocks. Your own block manufacturer’s specification should govern your choice.

Your skim coat primarily smooths the surface. My assessment is that cracks in the base may transmit through it, and a detached base can carry the skim coat with it. Treat crack-bridging capability as something that must be stated in the product’s technical data.

For your project, I would:

1. **Get the AAC supplier’s approved plaster specification.** A site-mixed render may be acceptable if they provide a formulation and application procedure for it.
2. **Check the skim coat’s data sheet** for permitted substrates, interior/exterior use, thickness and waiting time over fresh render.
3. **Follow the specified preparation, mesh and curing procedure.** Tropical sun and wind make protection during curing particularly relevant. Movement joints must continue through the finish; mesh does not replace them. [Hebel’s guide](https://hebel.com.au/wp-content/uploads/downloads/Hebel-PowerBlock-Design-and-Installation-Guide_HELIT043.pdf) covers both mesh and movement joints.

A manufacturer-specified additive could be a cheaper route to an acceptable site mix, but adding SBR or acrylic latex to an arbitrary mortar recipe does not establish AAC compatibility. [Sika’s SBR guidance](https://gbr.sika.com/en/distribution/building-supplies/pva-sbr-bonding/sikabond-sbr.html) describes its benefits for renders; the complete mix still needs approval for your substrate.

What country are you building in, what are the AAC and skim-coat brands, and are these interior or exterior walls? Also, has the cement/sand plaster already been applied? Those details would let me assess your proposed combination more specifically.