Looking For Watermarks

I am using Robinson lighter fluid, I have found some watermarks, but on other stamps that I should find watermarks I don't see them.

Does the lighter fluid brand make a difference, should I try Zippo? Should I break down and buy watermark fluid? Also how many times can a stamp be soaked and not damaged?
Thanks in advance!

Jackblack1, some watermarks are just very difficult to see. Usually, I use a Signoscope electronic WMK detector. If I can't see a watermark using this device, then I'll use the Fluid. In a small minority of cases, a watermark is just very difficult, or impossible to make out. It can happen with the same series of stamps. Sometimes in paper manufacturing, the pattern in the screen just doesn't thin the paper enough to make the watermark visible.

There have been discussions on Stamp Community forum about this subject. If you use the search link at the top of the page , you can find them.