This study systematically investigates the combined effects of wood species (Scots pine- Pinus sylvestris L., iroko- Chlorophora excelsa , and ash Fraxinus excelsior L.), heat treatment, and sandpaper grit size (100, 150, and 220), and varnish type (acrylic, polyurethane, and water-based) on surface roughness (Ra) and varnish adhesion strength of wood. Heat-treated and untreated specimens were conditioned at 20 ± 2 °C and 65 ± 5 % relative humidity, sanded with the specified grit sizes, and their surface roughness was measured according to TS 2495 EN ISO 3274. Subsequently, the specimens were coated with three different varnish systems, and adhesion strength was determined using the pull-off method in accordance with TS EN ISO 4624. The results demonstrated that sandpaper grit size was the most influential parameter affecting surface roughness, with Ra values decreasing significantly as grit size increased. Heat treatment increased surface roughness across all wood species. Accordingly, the highest Ra value (4.804 µm) was obtained in heat-treated Scots pine specimens sanded with 100 grit, whereas the lowest roughness value (2.252 µm) was measured in untreated ash specimens sanded with 220 grit. In terms of varnish adhesion strength, varnish type and wood species were identified as the most statistically significant factors. Acrylic varnish showed the highest adhesion performance for all wood species, followed by polyurethane and water-based varnishes. Increasing sandpaper grit size resulted in a consistent improvement in adhesion strength, with the highest values observed on surfaces sanded with 220 grit. Although heat treatment generally reduced adhesion strength, the magnitude of this effect varied depending on the varnish type. The highest adhesion strength (3.586 N/mm²) was recorded for untreated ash specimens coated with acrylic varnish and sanded with 220 grit, while the lowest value was observed in untreated Scots pine specimens coated with water-based varnish and sanded with 100 grit. • Sanding grit size was the main factor affecting surface roughness and varnish adhesion strength. • Heat treatment increased roughness and generally reduced adhesion depending on varnish type. • Acrylic varnish showed the highest adhesion across all wood species and preparation conditions. • Increasing grit from 100 to 220 improved adhesion by producing a smoother surface morphology. • Proper surface preparation–coating selection enhances durability of heat-treated wood.
Türkdal et al. (Thu,) studied this question.