How are rendering materials classified according to their particle size distribution in dry construction?

Solutions & Applications · Joint Treatment Materials

As we approach the final finish, the particle size decreases. The joint requires a durable material, trowelling requires a fine, spreadable material, and the final finish under oblique light requires a material with virtually no grain. The type of material and the number of coats required are determined by one factor: the Q1–Q4 quality standard required for the space.

Quality levels Q1–Q4

The finish quality of plasterboard is specified at four levels — the scale used by the German plasterboard industry, which is employed internationally in specifications. The grade is stated in the design or quotation; otherwise, everyone has a different idea of what ‘smooth’ means.

Level	What’s happening?	When is it sufficient?
Q1	Fill joints and screw heads with tape where required	Surfaces that are not visible or are to be tiled; second layer in double cladding
Q2	Q1 + a second layer at the joints, smooth transition to the slab	The standard: matt paints, textured wallpapers
Q3	Q2 + a wider finish at the joints and a thin trowel finish across the entire surface	Fine-grained matt paints, smooth wallpapers, without harsh lighting
Q4	Apply a continuous, even layer of the product across the entire surface using a trowel	Glossy and satin finishes, oblique light from windows or recessed lighting

Two families of materials

	Bagged (powder)	Ready-to-use (paste)
How they set	Chemical setting of gypsum — rapid strength development	Drying by water evaporation
Grain size distribution	Medium	Thin to extremely thin
Work	Construction joint · Q1–Q2	Trowelling and finishing · Q2–Q4
Products	Uniflott · Uniflott Waterproofing · Fugenfüller Leicht	F2F · Fill & Finish Light · Super Finish

Which material at which stage

Stage	Level	Material	Why
Joint with bevelled edges	Q1–Q2	Uniflott	High strength, low shrinkage — no tape at the ends HRAK/AK
Joint with tape, cut ends	Q1–Q2	Fugenfüller Leicht + tape	Economical, general-purpose
Joints in wet areas	Q1–Q2	Uniflott Waterproofing	Waterproof board → waterproof plaster
Jointing and finishing with a single material	Q1–Q4	F2F	Ready to use from the first coat right through to the final finish
Trowelling of large surfaces, ceilings	Q3	Fill & Finish Light	Lightweight, high coverage, easy to sand
Final finish under oblique light	Q4	Super Finish	Superb flatness, very easy to sand

Paper tape or mesh tape?

• Knauf Kurt paper-faced plasterboard — superior crack resistance, with a folded groove for internal corners. It is laid in a layer of plaster, so takes longer to install. Compatible with Fugenfüller.
• Self-adhesive mesh-reinforced glass fibre tape — applied directly to the joint, quick to install, ideal for repairs. For maximum strength, use with a chemically setting filler such as Uniflott.

The three-phase application

1. Joint treatment (Q1–Q2). Joint filling, tape where required, a second coat, and filling of screw heads. In the case of double cladding, the joints in the inner layer are also filled (Q1).
2. Trowelling (Q3). Prime with Tiefengrund to ensure uniform absorption, and apply a thin coat over the entire surface — not just in the joints.
3. Final finish (Q4). Very thin, continuous layer of Super Finish, sanding with fine-grit sandpaper, inspection with a lamp at an oblique angle prior to painting.

Flaws that become apparent after painting

Incorrect	Result
Finishing compound for construction joints	Crack in the joint within the first year
General-purpose plaster for damp-proof slabs	The joint becomes the weak point when exposed to moisture
Q2 under oblique lighting	The joints are ‘visible’ through the paint
Painting without a primer	Uneven gloss over joints and plaster
Application at temperatures below 10 °C or in damp conditions	Slow drying, shrinkage, cracks

Grouting — Bagged

Trowelling & Finishing — Ready-to-use

Membranes & Primers

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