Stress whitening is the pale, cloudy bloom that appears where styrene has been pinched, bent, or crushed — most often at a gate cut made too close, too fast, or with dull blades. The polymer chains literally deform and scatter light differently. It is a bloom, not a scratch: you cannot wash it off.
Prevention beats cure: two-stage cutting with a sharp single-blade nipper avoids most of it. When it happens anyway, light sanding removes shallow whitening, and on bare plastic a touch of heat or a gloss coat can reduce what remains.
See it in context in our nippers & cutting tools guides, or explore the tools hands-on in The Bench.