Checking the Substrate: What the Contractor Owes — and What They Don’t

Before the first roller touches the wall, there is an obligation — not an optional extra, but one defined in the VOB. Anyone who skips it remains liable later for defects they should have spotted. Anyone who documents it correctly is off the hook.

The duty to inspect is set out in the VOB, not in the small print

Painting and coating works are governed by ATV DIN 18363. Section 3.1.1 sets out the cases in which the contractor must raise concerns — literally, among others, for “unsuitable condition of the substrate, e.g. insufficient quality level, sanding or chalking render”.

The relevant technical guideline for this is BFS-Merkblatt Nr. 20, “Site-standard tests for assessing the substrate for coating and wallpapering works” (November 2016 edition).

Terminology note: since 2025, the Bundesausschuss Farbe und Sachwertschutz has referred to its guidance notes as BFS-Fachregeln and renames them in the course of revisions. Anyone citing them today names older Merkblätter by number and edition date, and newer ones as a Fachregel.

What “site-standard” means — and what it does not

The duty to inspect is expressly limited to defects that are visible or otherwise identifiable on site. Costly laboratory testing is not included. If a client demands more, this constitutes a Besondere Leistung (special service) and must be remunerated separately.

Also not owed: checking the substrate for dimensional tolerances under DIN 18202. BFS-Information 23-01 (October 2023) makes this explicit.

The individual tests

Finding	Site-standard test method
Cracks	Visual inspection, if necessary a water-wetting test
Soiling, fungi, algae, moss, efflorescence	Visual inspection
Moisture	Visual inspection, touch test, moisture meter
Chalking, sanding	Wipe test
Load-bearing capacity, adhesion	Scratch test, adhesive-tape test, cross-cut test
Absorbency	Water-wetting test

The scope of testing must be differentiated according to the substrate. BFS-Merkblatt Nr. 20 distinguishes between mineral substrates (render, concrete), timber, metal and plastics. For interior spaces, substrate-specific guidance notes are added — Nr. 8 for concrete surfaces, Nr. 10 for interior render.

The special case of filler compounds

For unknown filler compounds, BFS-Information 23-02 recommends an additional water-wetting test. If the compound softens spontaneously, it lacks load-bearing capacity — in which case concerns must be raised.

This is particularly relevant for refurbishments where nobody still knows what was applied twenty years ago.

Q1 to Q4 say nothing about the coating

A common misunderstanding: the quality levels Q1 to Q4 apply exclusively to the substrate surface to be produced — that is, to render or filler. They say nothing about the subsequent decorative or protective surface treatment.

And: checking the substrate for suitability remains the responsibility of the executing specialist firm even where a planner has specified the quality level. A planner’s specification does not provide exemption.

Raising concerns: in writing, without delay, beforehand

If defects are identified, concerns must be communicated to the client without delay under VOB/B § 4 Abs. 3 — ideally before work begins, and in writing. A verbal remark on site does not satisfy this formal requirement.

In practice, this means: a brief email with date, location, finding, photo, and recommended action. Two minutes’ effort that can make all the difference in a dispute.

The basic requirement, in one sentence

BFS-Information 23-01 sums it up: as a rule, all quality levels and render types are suitable for coatings, provided the substrate is dry, sound and load-bearing.

Everything else is detail work. But without these three properties, no product in the world will help — and this applies equally to an insulating coating and to an emulsion paint.

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Sources

• VOB/C ATV DIN 18363:2019-09, Painting and coating works, Section 3.1.1
• VOB/B § 4 Abs. 3
• BFS-Merkblatt Nr. 20, “Site-standard tests for assessing the substrate”, November 2016 edition
• BFS-Information 23-01 (October 2023) and 23-02 (August 2023)
• Landesinnungsverband des Bayerischen Maler- und Lackiererhandwerks, Info-Merkblatt “Untergrundprüfung im Maler- und Lackiererhandwerk”, June 2018
• Bundesausschuss Farbe und Sachwertschutz e. V., overview of the BFS-Merkblätter and Fachregeln, 2025 edition

This article reflects the state of the guidelines named at the time of writing and does not replace legal advice. The version in force at the relevant time is decisive in any individual case.