How to Mix Automotive Paint With Reducer & Hardener

by Paul MiceliUpdated July 02, 2023
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Items you will need

  • Paint mixing pot

  • Paint measuring stick

  • Automotive paint

  • Compatible hardener

  • Compatible reducer

  • Disposable stirring sticks

  • Cloth

  • Paint Gun

Mixing car paint can be a time consuming and confusing process. Modern automotive paints are activated with hardeners to promote faster drying times. Reducer is added to help the material flow through the spray gun easily. Mixing automotive paints is a vitally important aspect of body work. Adding too much or too little hardener will alter recommended curing periods, and the incorrect quantity of thinner can result in runs, sags or dry patches in the paint job. Here’s the quick and dirty on how to mix auto paint for refinishing your car.

Take a mixing pot and check inside to ensure there are no waste products from previous jobs. Contamination can lead to poor base coat adhesion and slow drying times. Wipe out the paint mixing cup with a clean piece of cloth to remove traces of dust and other foreign contaminants. Place the paint mixing pot on a flat, even surface. Stand the paint measuring stick against the inside wall so it rests in a vertical position.

Stir the automotive paint thoroughly with a disposable stirring stick before mixing begins. Make sure the separate pigments mix together. Check the lacquer against the auto body to make sure the color is accurate.

Refer to the technical data sheet supplied with the automotive paint to establish the correct amount of paint to hardener. Remember that the mixing ratio is written sequentially. So figures of 2:1:1, for example, will relate to two parts paint, 1 part hardener and 1 part of paint thinner. Check the markings on the paint measuring stick to ensure the correct ratio markings are available. Note that the type of paint will affect the ratio. Most modern two-pack systems and solvent-based metallic colors are mixed at a ratio of 2:1 with a compatible hardener or thinner. Water-based colors are usually mixed at a ratio of 10:1 with a compatible water-based thinner.

Determine how much unmixed material is needed for painting. Refer to the left-hand side of the paint stirring stick to view the different volume measurements. Note that each numbered vertical marking represents one-tenth of a liter of automotive paint. Add paint up to marking number 1 on the left-hand side of the stick if 100ml of unmixed paint is required. Add up to the number 2 if 200ml of unmixed paint is needed. Add up to any other number that represents the correct volume of material necessary to complete the job. You can use measuring cups if you feel it necessary. You can opt for a single stage paint to eliminate the need for a clear coat or go through with a two stage painting process. Two stage products require an activator and a sealer topcoat. No matter what you opt for, be sure to level all paint to avoid the orange peel effect.

Take a tin of compatible hardener and look at the markings on the center of the paint measuring stick, which represent the second part of the mixing ratio. Fill the paint mixing pot to the same number on the center of the stick as you used when adding unmixed paint. Add to the number 1 if the paint was added to number 1 on the left-hand side; 2 if the paint was added to 2 on the left-hand side; or any other relevant number that matched your original paint quantity.

Repeat the process on the right-hand side of the stick with a compatible reducer, making sure the pot is filled to the same number on the paint measuring stick so it matches the number used for the unmixed paint and hardener. Allow the material to settle for a few seconds before stirring the three components together and adding the mixed product to a spray gun.

Viscosity Cup Method

  1. Refer to the technical data sheet and view the recommended viscosity times. A viscosity cup holds a standard measurement of thinned paint and the time represents how long it should take for thinned material to drain through the cup. This is usually presented as range. For example, the viscosity time may be in the range of 15 to 18 seconds.

Activate the paint by adding a quantity of hardener. See the technical data sheet to establish the ratio and use a marked paint mixing stick to help you create a base material before the viscosity check begins. Ensure that you have a stopwatch ready and that the timer is set on zero.

Fully immerse the viscosity cup into the activated paint, ensuring the removable cap on the cup is in place. Wear latex gloves to stop paint getting onto the skin. Lift the viscosity cup out of the paint and allow excess material to drop back into the tin.

Remove the cap on the viscosity cup and, at the same time, activate the stopwatch. You may require a second person to make this easier. Allow all of the material to flow through the hole of the viscosity cup until it is empty. Deactivate the stop watch as the last of the paint falls through.

Read the time on the stopwatch. If it falls within the recommended range, the paint is ready to use. If the time recorded is below the recommended range, the mixed paint is too thin and more base color should be added. If the time is above the recommended range, the paint is too thick and more thinner should be added. Continue to carry out viscosity checks until the correct times are achieved.

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