In the context of electrostatics, solvents are divided into two categories based on their polarity: polar and non-polar. The variation in polarity among different solvents is utilized to modify the resistivity (electrical resistivity) of the paint compounds. Non-polar solvents have poor sprayability. These solvents encompass aromatic and aliphatic hydrocarbons, chlorinated solvents, and terpenes. The electrostatic sprayability of primary coating materials can be enhanced by the addition of suitable polar solvents. On the other hand, polar solvents include ketones, alcohols, glycol ethers, esters, and nitroparaffins.

Viscosity guide:
The initial paint formulation should possess a substantial viscosity (ideally exceeding 50 seconds on Ford Cup #4) to ensure the solids content remains within acceptable limits following the addition of solvents. Typically, it is more advantageous to modify the viscosity following the adjustment of resistivity, as the viscosity plays a lesser role in the electrostatic spray ability.

How to adjust electrical resistivity in paint formulation:
1- Formulate the paint sample with high viscosity, preferably over 50 seconds with Ford Cup #4.
2- After complete mixing, measure the electrical resistivity of the undiluted paint with one of the following devices available: Paint Test Equipment (01-76652), Paint Test Equipment (02-76652), Paint Test Equipment (03-76652), Paint Test Equipment(04-76652), or Paint meter (00-70408). Adhere to the test procedure instructions while utilizing the above-mentioned equipment. Record the value of the resistivity. If the resistivity exceeds the value indicated in Figure 1, proceed to steps 3 and 4. If the quantity is smaller, proceed to step 5.
3- Add the minimum amount of reference polar solvents to reach the desired resistivity. Several tests may be necessary at this stage.
4- Adjust the viscosity by adding the smallest amount of non-polar solvent to obtain the desired film flow characteristics.
5- If the resistivity measured is less than the desired value, add a non-polar solvent to adjust the resistivity.

Paint resistance control by solvents: Bulking agents consisting of non-polar solvents can be employed to modify the viscosity or fluidity of paint, while minimally affecting the electrical resistivity of the mixture. This rule does not apply to paints with low electrical resistivity, such as vinyl solutions and nitrocellulose materials. The addition of non-polar solvents can occasionally decrease the conductivity of these unique compounds. In general, the inclusion of highly polar solvents significantly decreases the resistivity of the mixture, while solvents with moderate polarity have a relatively smaller effect on resistivity. Therefore, there is an inverse correlation between the polarity of the solvent and the electrostatic resistivity of the paint.

Exceptions:
A solvent with a specific polarity may reduce the paint resistivity more than a more polar solvent due to the complex reactions between solvents, resins, and binders.