In oil-based paints, which is the most commonly used thinner for adjusting brushability and viscosity on site? (Choose the solvent typically recommended for oil paints.)

Introduction:
Oil-based paints require compatible thinners to modify viscosity, flow, and leveling. The question asks for the typical field-standard thinner used with traditional oil paints and primers.

Given Data / Assumptions:

• Binder system: drying oils/alkyds in oil paints.
• Goal: improve brushability and penetration without harming film formation.
• Common site solvents: turpentine and petroleum distillates.

Concept / Approach:
Turpentine (gum or mineral turpentine depending on locale) has long been specified for oil paints due to compatibility with drying oils and favorable evaporation rate, promoting good film without excessive solvent retention.

Step-by-Step Solution:

Verification / Alternative check:
Manufacturer data sheets for traditional oil/alkyd paints specify mineral turpentine/white spirit for thinning and cleaning, confirming ubiquity.

Why Other Options Are Wrong:

• Naphtha: usable in certain cases but not the default standard across all oil paints.
• Both equally standard: overgeneralizes and ignores system-specific guidance.
• Neither: contradicts established paint practice.

Common Pitfalls:
Over-thinning which reduces hiding power and film thickness; mixing incompatible solvents; ignoring maximum thinning percentages on data sheets.

Final Answer:
Turpentine