Short and simple, but I suspect the answer won’t be.
How does regular paper turn translucent when it gets greasy, or comes in contact with oil?
Spiffo
A block of ice is transparent, but if you shave or grind it it becomes almost opaque. The basic property of the ice hasn’t changed, but by breaking it up into small pieces you create lots of surfaces where light can reflect and scatter. It’s these scattering surfaces that make the glass of shaved ice look white. Now pour water into this glass and it becomes almost transparent again, not just because some of the ice melts, but because the water-ice boundaries reflect/scatter much less light than air-ice boundaries. You’re filling up the gaps and reducing reflection. My WAG (but I’m pretty confident) is that the same thing happens with paper. Individual fibers are fairly transparent, and oil fills up the gaps to reduce scattering/reflection.
Isn’t it obvious that you have to search for “Wet T-shirts” to find a thread explaining it? 