It looks as though masking fluid produces the same effect as wax? One of my sons used to experiment with it, both for painting on heavy parchment-like paper and on sheets of paperbark. He would draw outlines on the paper with a bit of hard candlewax, then paint over it with watercolours. With the paperbark, which is quite soft on the inner side and easy to tear, he would rub in soft raw beeswax from my bees, which made a more blurred/soft effect but didn't tear the paperbark, then paint over it.