What's the mix ratio, and do I measure by weight or by volume?
The ratio is printed on the product's TDS and on the container. The most important thing to know: by-weight and by-volume ratios are often different numbers for the same product because Part A and Part B have different densities. Always confirm which one you're reading.
- 1A:1B by volume products (like many platinum silicones and Smooth-Cast resins) are the most forgiving, equal size measurements.
- By-weight ratios (common on urethanes and foams) require an accurate scale. A kitchen scale to 0.1 g is usually fine for small batches.
- Never "eyeball" platinum silicones or urethanes. Off-ratio is the #1 cause of soft, tacky, or uncured results.
Most common mistake: Mixing thoroughly matters as much as the ratio. Scrape the sides and bottom of the cup, then pour into a second clean cup and mix again ("double potting"). Unmixed material clinging to the original cup is a frequent cause of uncured spots.