What should be applied first when treating most spots, especially those with unknown soils?

When treating spots, especially with unknown soils, the application of a volatile dry solvent is often the preferred method. This is because volatile dry solvents are specifically designed to dissolve oils and greases that may be present in the stain without introducing moisture that could potentially spread the stain or cause additional damage to the upholstery.

Using a dry solvent first allows the technician to assess how the fabric responds to the treatment and whether the stain is oil-based or water-based. This step is crucial, especially with unknown soils, as it helps in determining the right follow-up treatment without the risk of setting the stain further into the fabric.

Other options like water solution or rubbing alcohol could potentially worsen some stains or damage certain fabrics, particularly if they are sensitive to moisture or chemicals. The use of a stain remover should occur after identifying the nature of the stain, ensuring the right product is used for effective remediation. Thus, starting with a volatile dry solvent ensures a safer and more targeted cleaning approach.