**Your concern is justified: a premixed skim coat cannot be relied on to protect an unsuitable cement–sand base plaster from cracking.** However, site-mixed plaster is not automatically unsuitable for AAC.

AAC needs a compatible render because:

- **Dry blocks can absorb mixing water too quickly**, impairing the plaster’s curing and adhesion.
- **Cement-rich plaster can shrink excessively**, causing cracks or separation from the blocks.
- **Movement at openings, concrete-frame junctions and movement joints** needs appropriate detailing; additives alone cannot accommodate it.

Hebel’s technical guidance explicitly allows site-mixed render, but warns against strong, cement-rich mixes and calls for compatible strength, adhesion and vapour permeability. It also specifies mesh at vulnerable locations and warns against rendering across movement joints. [Hebel technical manual, pp. 9.1–9.3](https://hebel.com.au/wp-content/uploads/2016/11/Hebel-Tech-Man-Pt5-Fixings-Finishes-Access-2.pdf)

**Polymer modification helps, but it is not a guarantee against cracking.** Suitable additives can improve adhesion, toughness and flexibility. Site-mixed polymer-modified render is possible—for example, Sika supplies SBR additives for renders—but a generic additive recipe is not proof that the resulting mix suits your particular AAC. Get an AAC-approved specification covering proportions, preparation, thickness and curing. [Sika’s render-additive guidance](https://ind.sika.com/en/construction/concrete-repair/site-batch-mortars/sikalatex-sbr.html)

Your skim coat primarily provides a smooth finishing surface. Some products can cover minor surface cracks, but that is different from preventing future cracks in the underlying plaster. My practical assessment is that cracks may reappear through the skim, and a skim cannot rescue a base coat that loses adhesion. Weber’s guidance treats skim coating as a finishing step alongside correct mixing and curing. [Weber Thailand guidance](https://www.th.weber/en/translate-th-masonry-wall-preparation-products/perfectly-smooth-finishing-leaving-nicely-trouble-free-walls)

If plastering has **not started**, I would spend the extra money on an AAC-approved base render unless your block supplier can provide a proven site-mixed alternative. The base coat is where compatibility matters most. In tropical conditions, protect fresh render from direct sun, wind and heavy rain, and follow the specified curing procedure. [Sika application instructions](https://ind.sika.com/en/construction/concrete-repair/site-batch-mortars/sikalatex-sbr.html)

If it is **already applied**, assess its bond and any cracking before skimming; ordinary plaster does not necessarily need removal simply because it was mixed on site.

What country are you building in, what is the skim-coat product, and are these interior or exterior walls? Those details would help determine whether your proposed combination is suitable.