Paint thinners differ according to the paint formulation being thinned. Oil-based paints are not water soluble and therefore require a mineral oil-based thinner. On the rare occasion that a solvent-based coating requires thinning, such as when the first coating of floor enamel or of rustproofing paint applied to a wood surface requires a thinned application for improved penetration, a solvent-based paint thinner does the job. Water-based paints, including aqueous acrylics, require a water-based thinner. All three thinners may themselves be diluted, but each requires its own dilution medium.
Determine the type of the paint thinner you intend to dilute. The thinner may be mineral oil-based, petroleum- or aqueous-based acrylic or water-based. Read the label or contact your paint store to be certain.
Dilute a small amount of thinner in a glass flask and stir with the glass rod. Dilute water-based and aqueous acrylic-based paint thinners with distilled water. For mineral oil-based paint thinners use vegetable oil or walnut oil. Petroleum-based paint thinners require isopropyl alcohol to dilute the product.
Test your diluted thinner with a sample of the paint it will be used to thin. Note the thinning and drying characteristics of your formulation before mixing and applying it on a large scale. Adjust your formulation as required, retest and mix a larger batch of diluted thinner as needed.