The application of gloss reduced coatings such as matt or satin polyurethane floor coatings, is more demanding than application of high gloss coatings. Gloss reduction of polyurethane coatings is mainly achieved by adding flatting agents into the system. Once the film has dried, the flatting agents provide micro roughness on the surface, which appears as a gloss reduction of the coating.
The degree of matting effect also depends on the thickness of the film. Most appearance imperfections such as gloss variations, lap marks etc are a result of uneven film thickness.
The final gloss level of a coating system is influenced by
- Coating material
- Environmental conditions
- Application technique
Incorrect application technique or inappropriate environmental conditions can result in appearance defects such as:
- Brush marks
- Roller marks
- Lap marks
- Gloss variations
1. Coating material
- Shaking/stirring of material - All pigmented coatings, including matt and satin polyurethane coatings, must be thoroughly shaken or stirred for at least 3 minutes before use to ensure that the flatting agent is distributed evenly. Improper shaking of the material will typically cause patchy and streaky finishes;
- Straining - Material should always be strained through a stocking to remove any reacted particles that may have formed around the lid;
- Use only compatible materials - It is always advisable to use an integrated coating system from one manufacturer;
- Never mix coatings from different batches on one job - All reduced gloss coating systems naturally have very slight gloss differences between batches and
- Mixing matt and gloss - A satin finish provides a gloss level that is higher than matt but lower than gloss. Matt and gloss coatings can be mixed to provide a satin coating. Always follow the manufacturer’s instructions with respect to mixing.
2. Environmental Conditions
- Temperature - Recommended application temperature is between 12° C and 30° C. Coating at lower temperatures can result in moisture condensation on the surface, producing a milky or bubbled appearance. Coating a floor subject to patches of sunlight can result in uneven gloss. An easy precaution is to cover windows with newspaper;
- Humidity - One pack polyurethanes are moisture cured coatings, so their drying behaviour depends on the humidity of the surrounding air. The orientation of matting agents and settling will be influenced by changes in humidity and
- Air flow - Uncontrolled air flow over a freshly applied coating can cause irregular drying, leading to gloss variations or bubbling. Dust will adversely affect the appearance of the finish. Uncontrolled airflow combined with high humidity is particularly likely to cause finishing problems.
3. Application Technique
The gloss level of pigmented systems depends on the application method. One coating material applied by different methods can give different gloss levels. Problems with gloss variations such as lap marks and roller marks are highlighted on darker timbers, or if the material is applied across the grain.
- Tools - Best results have been achieved by using a 6.3mm nap mohair roller. Edges can be brushed. Lambswool applicators are not recommended due to their tendency to produce film thickness variations and
- Application - Wherever possible, all coatings should be applied in the direction of the boards. Always apply the coating materials in a single direction. Try to move lap marks between the boards where they are less visible. It is important to keep a wet edge at all times during application.
Reference: Toby Technical Bulletin TB010