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D358-98 – Standard Test Method Technical Guide
This standard specification, designated D358-98 (Reapproved 2012), provides guidelines for selecting wood panels used in weathering tests of coatings. It aims to minimize variability in test results by specifying wood species, density, and character requirements for exterior solvent-borne or water-borne paints and similar materials, applicable to both outdoor and accelerated laboratory tests.
🌲 Species Specification and General Considerations
The designated material for test panels is western red cedar (Thuja plicata). A variety of wood species are recognized due to wide variations in anatomy and density, which significantly influence finish performance. Latewood band width and density are particularly critical, with cedar and redwood having lower proportions of hard summerwood in narrow bands and containing extractives that may discolor paint films, while southern pine has a larger proportion of hard summerwood in wide bands.
Moisture content is also key: some industrial processors may use wood at higher moisture levels for factory-applied finishes, but standard equilibrium moisture conditions are recommended for general testing.
📏 Weight, Density, and Moisture Conditions
The density of wood panels is specified under controlled conditions to ensure consistency. When equilibrated per standard conditioning or the D3924 specification, the wood must meet the following weight-per-volume requirements:
| 🟦 Condition | 📏 Density (kg/m³) | 📐 Density (lb/ft³) | ⚡ Relative Humidity | 🌡️ Temperature |
|---|---|---|---|---|
| Standard conditioning | 350–370 | 22–23 | 60–65% | 21 ± 0.5°C (70 ± 1°F) |
| D3924 standard conditions | 343–359 | 21.4–22.4 | 50 ± 5% | 23 ± 2°C (73.5 ± 3.5°F) |
💡 Tip: The density requirement eliminates wood specimens that vary greatly from the species average. At 60–65% relative humidity, wood has approximately 12% moisture content, closer to exterior exposure conditions, while at 45–55% it contains 9–10%.
📐 Wood Character and Grain Orientation
The wood must be heartwood with edge grain, free of knots and other defects, and have uniform color without excessively light or dark streaks. At least ten annual growth rings per inch along the log radius are required. Grain orientation relative to the surface is critical:
| 🟦 Grain Angle with Surface | 📐 Classification |
|---|---|
| Less than 30° | Flat-sawn or flat-grained |
| Between 30° and 60° | Bastard-sawn |
| Greater than 60° | Quarter-sawn, edge grain, or vertical grain |
⚠️ Warning: The angle between growth rings and the specimen surface has a tremendous influence on finish performance. Using improper grain orientation can lead to misleading test results, so always verify the cut type per this classification system.
❓ Frequently Asked Questions
🔍 What species of wood is specified for test panels?
Western red cedar (Thuja plicata) is specified to minimize the influence of wood variation on coating weathering tests.
💡 Why are density requirements important in this standard?
They eliminate specimens that deviate significantly from the average density of the species, ensuring consistent and comparable test results.
⚡ What is the required number of growth rings per inch?
The wood must have at least ten annual growth rings per inch along the radius of the log from which it was obtained.
📌 How does grain orientation affect finish performance?
Grain angle influences paint adhesion and durability; flat-sawn boards (angle <30°) behave differently from quarter-sawn (>60°), so classification is essential for reproducible testing.