Contents
The 4:1 or 5:1 you see on the label of a 2K acrylic primer is one of the most important instructions governing the product's performance. This ratio is not a matter of preference but the point at which the formulation is chemically balanced; deviations made by eye come back as drying, sanding and adhesion problems. In this article we explain what the ratio means, how to measure it correctly and what happens when the mix is wrong.
What Does the Mixing Ratio Mean?
A 4:1 ratio (often written as 4+1 in the workshop) means adding 1 part hardener to 4 parts primer by volume. With a 5:1 ratio, 1 part hardener is added to 5 parts primer. A part can be one graduation on a scale, one measuring cup or any unit of volume; what matters is that the ratio is maintained.
In practical terms:
- In a 4:1 mix, the hardener makes up 20% of the combined primer and hardener.
- In a 5:1 mix, this share is approximately 16.7%.
The table below shows the volumes for commonly prepared quantities:
| Mix to prepare (excluding thinner) | 4:1 primer + hardener | 5:1 primer + hardener |
|---|---|---|
| 300 mL | 240 mL + 60 mL | 250 mL + 50 mL |
| 600 mL | 480 mL + 120 mL | 500 mL + 100 mL |
| 900 mL | 720 mL + 180 mL | 750 mL + 150 mL |
Thinner is not included in this ratio and is specified as a separate percentage, for example “4:1 + 10–20% thinner”.
Why Is the Ratio Specific to the Formulation?
In 2K acrylic primers, the binder is an acrylic resin carrying hydroxyl (OH) groups, while the hardener contains isocyanate (NCO) groups. Curing creates a cross-linked network formed by the reaction between these two groups. At the formulation stage, the amount of OH in the resin and the amount of NCO in the hardener are calculated, and the ratio at which the two are in balance is determined.
This means that:
- A 5:1 primer is not a product that “hardens less” than a 4:1 primer; it has a different resin content and solids content and is set to its own balance.
- The same hardener can be used at different ratios. For example, ZN-410 2K Acrylic Primer Hardener is used with both ZN-210 4+1 Acrylic Primer Filler and ZN-220 5+1 HS Acrylic Primer; the difference in ratio comes from the primer's formulation.
- Applying one primer's ratio to another primer, or working on the principle that “a little extra hardener dries it better”, upsets this balance.
In HS (high-solids) primers, the higher solids content means more fill per coat and less sinkage after drying.
Graduated Cup or Scales?
With a Graduated Mixing Cup
Because the ratios are given by volume, the most practical method is a graduated mixing cup printed with ratio scales:
- Stir the primer thoroughly in its tin, right down to the bottom; the filler pigments settle during storage.
- Choose the correct scale on the cup (4:1 or 5:1) and pour in primer up to the relevant line.
- Add hardener up to the next line on the same scale and mix thoroughly.
- Add thinner up to the relevant percentage line, mix again and strain the mix into the spray gun.
Reading the wrong scale is one of the most common mistakes, and it throws the ratio off without anyone noticing.
With Scales
Scales are a precise method, but there is an important pitfall: a ratio by volume is not the same as a ratio by weight. Filled primers have a considerably higher density than the hardener. For example, if a primer with a density of 1.6 kg/L and a hardener with a density of 1.0 kg/L are mixed 4:1 by volume, the ratio by weight is approximately 6.4:1. When working with scales, use only the weight values given in the product's Technical Data Sheet (TDS); never apply the volume ratio directly on the scales.
Typical Values: 4:1 and 5:1 Primers
| Property | 4:1 acrylic primer filler | 5:1 HS acrylic primer |
|---|---|---|
| Mixing ratio (by volume) | 4:1 | 5:1 |
| Hardener share of the mix | 20% | ≈ 16.7% |
| Thinner addition | 10–20% | 10–20% |
| Pot life (20 °C) | 30–60 min | 30–60 min |
| Number of coats | 2–3 | 2–3 |
| Flash-off between coats | 5–10 min | 5–10 min |
| Total dry film thickness | 60–100 µm | 80–150 µm |
The table shows that the difference in ratio does not fundamentally change the application parameters; the real difference lies in the solids content and the filling capacity per coat. The values are typical; for exact figures, refer to the technical data on the product pages and to the TDS.
Pot Life and Thinner Addition
The reaction starts the moment the hardener is added. Pot life is the time during which the mix remains at an applicable consistency, and it is directly related to temperature: roughly speaking, every 10 °C rise shortens the pot life markedly, often by half.
- Only mix as much as you will use within the pot life.
- Do not try to thin down a mix that has started to thicken; the reaction has already progressed and the film properties will suffer.
- Do not use hardener that has been exposed to moisture, has turned cloudy or has gelled; curing will be impaired even if the ratio is correct. Close the tin tightly after every use.
- Choose the thinner ratio from the range in the TDS according to what the primer is being used for. With primers suitable for wet-on-wet use, apply only the ratio specified in the TDS for that use.
- Whether the thinner percentage is calculated on the primer alone or on the combined primer and hardener can vary from system to system. The thinner lines on graduated cups do this calculation for you; if in doubt, check the TDS.
- For thinning, use a thinner specified by the system, such as ZN-510 Acrylic Thinner. Cellulose (nitro) thinners are not suitable for 2K systems; any alcohol and moisture they contain reacts with the hardener and impairs curing.
Hardeners contain isocyanates; during mixing and application, use the respiratory protection, gloves and eye protection specified in the product's Safety Data Sheet (SDS).
Tip: Write the time you prepared the mix on the cup. On hot days in particular, limiting the amount you prepare to the area you will prime in one go reduces both material waste and quality risks.
Consequences of the Wrong Ratio
Too Little Hardener
- The film dries slowly and stays soft; it clogs the abrasive.
- Solvent resistance drops; solvents from the topcoat swell the primer, causing sinkage and making sanding marks more visible.
- Water and moisture resistance is weakened, and the long-term risk of blistering increases.
Too Much Hardener
- Unreacted isocyanate can react with moisture in the air and release carbon dioxide; this may show up as micro-bubbles and pinholes.
- The film becomes brittle, and the risk of cracking under impact and flexing increases.
- Intercoat adhesion may be weakened, and material is wasted unnecessarily.
In both cases, the product cannot deliver the performance defined in its TDS. Because the problem often only appears weeks or months later, the cost of mixing without measuring can go far beyond a single job.
If you notice before application that the mix has been prepared incorrectly, the safest course is to prepare it again rather than trying to correct it. With a primer that has already been applied at the wrong ratio, wait for curing to complete and then assess the surface; a film that stays soft or blisters may require the system to be rebuilt from that layer upwards. For short definitions of terms such as pot life, solids content and cross-linking, take a look at our glossary.
Summary
4:1 and 5:1 express the chemical balance of two different primer formulations; neither is “stronger” than the other. The ratios are by volume; if you use scales, go by the weight values in the TDS. Use the correct scale, respect the pot life, choose the system thinner and never add hardener by eye.