Finishing
Abrasion is permanent; disorientation is not. A press restores a step, which is why the care label is part of the claim.
— AATCC
Before any heat reactivation
A linear decay is fitted from two points and it is a convenience rather than a mechanism: real spray rating falls fastest in the first few washes as loosely held finish leaves, then more slowly, so a rate fitted over five washes will be pessimistic at fifty and one fitted over fifty will be optimistic at five. Fit across the range the claim covers. The rating scale itself is ordinal and coarse - the rungs are 0, 50, 70, 80, 90 and 100, and nothing lies between them - so the continuous figures here are an interpolation used to compare and project, and the reported values are what an operator would actually write down. Reactivation gain is a property of the chemistry and the care applied: a modern non-fluorinated repellent typically recovers less than a long-chain fluorocarbon did, and a low-temperature tumble recovers less than an iron. Measure it rather than assuming the default. Nothing here addresses the durability of the substrate itself: a fabric whose surface has been abraded or pilled by laundering will spray-rate poorly however intact its chemistry, and that damage does not reactivate.
Spray Rating Decay & Heat Reactivation — free, with the formula and a worked example, at Textile School.