Brazil Scott 661B On Cover

I have a thin airmail cover with a block of 611 or 611b. I am trying to view the watermark on cover. I flooded the block with Ronsonol and I can see through, but still not able to see the watermark.
Anyone have a method on how to view a watermark through a thin airmail cover?
Thanks, RT

What you did but with a strong light over just the stamp while you look from inside the envelope. Do not have the light shining in your eyes, just the envelope. The new fangled small high lumen flash lights work well.

Thanks for the tip. I was able to see the WMK by placing a piece of tissue inside the envelope to hold it open. Room was dark except for a table lamp pointing away to my right. Fluid did not help, so I was doing this on a dry envelope viewing from the front.
I held the flash light pointing inside the envelope at an angle and slowly moved the envelope away from the light while staring at the stamps. An "O" appeared and while I continued to stare I saw more letters appear. It still took awhile but I was able to identify "COR", the WMK is 267. I could not see a star or plus.
It's kind of like the puzzles oline that you stare at to find the image of Jesus!