Only proper pretreatment ensures perfect results


The demands on the economic efficiency and perfection of your work are constantly growing. In many cases, the requirements to be met are becoming more and more demanding. Difficult substrates have a great impact on the successful outcome of your work.


Today, you’re confronted with a great variety of different surfaces, including a large number of screeds, concrete and cellular concrete, various substrates made of wood, tiles, rigid PVC and many more. Luckily, Thomsit always offers a suitable, economical and safe primer for you to meet all these requirements.

Undercoats, primers and moisture barriers

Undercoats and primers form a bonding bridge between the substrate and the levelling compound or adhesive applied on top. Their use is prescribed by DIN 18365 "Flooring Works". It is always necessary to follow the manufacturer's instructions for use.

DIN 18365 "Flooring Works"

In paragraph 2.9, the standard specifies:

"Primers, fillers and levelling compounds must bond firmly and permanently to the substrate, provide a base for the adhesive and be designed in such a way that the floor covering lies on top without any dimensional change. They must not adversely affect the substrate, underlay, adhesive and floor covering."

One of the most important steps in floor construction is the pretreatment of the substrate with suitable undercoats and primers. To ensure the full functionality of these products, it is first of all necessary to prepare the substrate by mechanical means.

Old, non-absorbent floors, for example, first need to be cleaned in order to completely remove old, adhesion-inhibiting care films. As a rule, basic cleaners such as CC-Dr. Schutz Basic Cleaner R are used in combination with a grinding machine equipped with a black stripping pad (cleaning fleece). Thorough rinsing with clear water always completes the cleaning process.

Old subfloors

Some old subfloors, however, may require more intensive pretreatment. This is for instance the case when the unstable residues of flooring installation products or old floor coverings need to be removed. This is usually done with the help of screed grinders, shot-blasters or other special tools. The manufacturer should be contacted in advance of the work to determine which tool is most effective for the task at hand. If in doubt, carry out preliminary tests.

Newly installed screeds

For new screed surfaces, machine sanding is also indispensable. It is done with an abrasive disk of medium grit (e.g. 60 or 80 grit) or suitable sanding screens. This is the only way to remove the unavoidable soiling (e.g. cement slurries, dusty peripheral zones) caused by the work of other craftsmen. Failure to remove this soiling would prevent a secure bond between the subsequent layers. The standard pretreatment always follows the same scheme:




Wooden floorboards

The same applies for wooden floorboards. For structural reasons, however, they first need to be secured with wood screws before sanding and vacuum-cleaning them. The primer of choice is Thomsit R 790 Filling Primer since it combines two steps in one operation: it fills the joints and primes the surface.

Retighten the screws                        

Sand the boards                   

Vacuum-clean the surface        

Fill the joints           

Old tiles

Smooth subfloors of poor adhesion require proper pretreatment with suitable undercoats and primers. Only then can the levelling compound be applied on top. Application of a levelling compound on a ceramic tile surface would not be possible without prior use of a primer. The adhesion-promoting effect of products like Thomsit R 766 Multi-Purpose Primer plays an important role.

Moisture barriers


Besides their adhesion-promoting properties, primers can also be used to protect the substrate against moisture – provided they have been specially designed for this purpose (see information in the Technical Data Sheet). These primers are reaction resins based on a 2-component epoxy resin (Thomsit R 755) or a 1-component polyurethane (Thomsit R 740).


Primers allow the floor layer to apply a moisture barrier on waterproof, mechanically prepared substrates. This barrier, in turn, protects the subsequently applied flooring installation products from the negative impact of moisture rising from the substrate.

Always apply Thomsit R 740 and Thomsit R 755 in two coats when using them as moisture barriers. Immediately after completion of curing on the following day, the primed substrates are ready to receive flooring.


After curing, the above-mentioned reaction resin primers provide a relatively smooth surface.

To ensure a firm bond with the subsequently applied levelling compound, the surface must now be treated with a black stripping pad to produce a surface with good keying properties. After that, apply an intermediate priming layer with undiluted Thomsit R 766 Multi-Purpose Primer. Especially when expecting higher stresses or applying the levelling compound in thick layers of more than 10 mm, it is necessary to sprinkle quartz sand (Thomsit Quartz Sand QS 10, grain size 0.4 - 0.8 mm) into the last coat of reaction resin while it is still fresh. Please refer to the Technical Data Sheets of the respective products for more detailed information.

If in doubt, request advice from our technical advisors.

Furthermore, primers can be used to protect moisture-sensitive substrates or old flooring installation products (as long as they firmly adhere) against the detrimental effect of the gauging water used for mixing the levelling compound. Being a 1-component, water-free primer, our Thomsit R 740 offers easy handling and rapid work progress. Most important precondition: Follow the manufacturer's instructions.

Not only moisture but also certain pollutants can be controlled in this way. Thomsit R 755, for instance, can be used to renovate PAH-contaminated parquet floors quickly, reliably and durably (PAH stands for polycyclic aromatic hydrocarbon). Information on materials and procedures is provided by your local Thomsit advisor. Please contact him to obtain expert advice for your specific application.

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