In oil-based paints used for buildings and metalwork, which solvent/thinner is most commonly used to adjust brushing/spraying viscosity and aid drying? Select the best traditional choice.

Introduction / Context:
Oil paints are formulated with drying oils (e.g., linseed), pigments, and volatile solvents to achieve workable viscosity and film formation. Field painters universally use a standard thinner to condition these coatings for brush or spray. This question targets that practical solvent choice.

Given Data / Assumptions:

• We refer to oil-based (not water-borne) paints.
• Solvent should be compatible with oil vehicles and resins.
• Traditional practice is considered.

Concept / Approach:

Turpentine (or mineral turpentine substitutes/white spirit) is the common solvent for oil paints. It lowers viscosity for application, improves flow and leveling, and evaporates at a controlled rate to allow oxidative curing of the film. Alcohol (spirit) is not suitable for oil vehicles; water cannot thin oil paints; coal tar and generic petroleum fuels are inappropriate solvents.

Step-by-Step Solution:

Verification / Alternative check:

Paint manufacturer datasheets specify turpentine or mineral spirits as recommended thinners/cleanup solvents.

Why Other Options Are Wrong:

Alcohol/water do not dissolve oil vehicles; coal tar/petroleum fuel fractions are unsuitable and unsafe for consistent paint performance.

Common Pitfalls:

Confusing solvent roles in oil vs spirit/latex systems; over-thinning which can reduce hiding and gloss.

Final Answer:

Turpentine