I'm a big fan of Schmincke products. I like the Aero colours and their Horadam watercolours. So when I saw this Aero Opak Medium that turns transparent paints opaque, I were sold and ordered a bottle from our local distributor. It only took 3 months to get here but that was okay. I could not beat the price no matter where I bought it overseas. They have in the meantime, came out with an opaque line which I think can be interesting as well. The most transparent paints I have are my E'tac Marissa colours. I used the Arylide yellow and Forrest green. I drew a white line on the black paper, which I sealed by the way, with a paint marker. I then filled the cup with 6 drops of paint and layed everything on the paper. As you can see it changed the white but almost nothing on the black. I then made up more cups with 4 drops of paint and 2 drops of Aero Opak Medium. They recommend not to go over 50% in the mixture. I could already see the colour change in the cup. I sprayed it over the line on the paper. Firstly it lays down a bit dark but then it flashes and turns to a milky pastel colour. So I thought, this stuff basically buffers the colours just like white would. I then sprayed just the medium to see what it does. Just as I thought. It is white after it flashes off but the interesting part is that you can smudge it just like you do with soft pastels. It looks to me like it is just a white pigment in some kind of binder that flashes off and just leaves the pigment behind. Conclusion I don't think I will use it with my colour paints as I do not buffer my colours. Adding it to white to make it more opaque could be a good idea though.