I've read some good reviews by SCF members about specialty devices, but I haven't tried any yet. For me a black tray works well and for the most difficult ones, I use wet scanning. As for fluid, I haven't tried anything but Ronsonol which hasn't failed me yet.
To me the hardest part about learning to see watermarks was knowing what to exactly look for. Below is an illustration that shows a basic idea of what you see when watermarking. Light from your lamp above you or from the scanner head penetrates the stamp. The arrows represent the light reflected back which you see. A watermark is basically an area of the paper that is thinner than the rest. This causes that area to appear darker, whether it be the ink of the design or the combined cancel ink and black background.
So let's put what we now know to work on an #416. It's perf 12 so we know that it has to have a Single Line (SL) watermark. It is also yellow which are considered the most difficult color stamps to see watermarks on, as well as orange.
Here's the SL watermark zoomed in:
Can you find areas on it that correlate to each of the 5 arrows in the illustration above? Post an image as a reply with the arrows numbered 1 through 5 from left to right.
Wet scanning has never failed me. Do be careful and check that your watermarking fluid won't damage your scanner or leach ink from the black paper that you use as a background. Test small areas of the scanner lid to see if the plastic softens. Touch a paper towel to your soaked black paper to see if any black ink transfers to it.Precaution:
Non-flammable fluids like Clarity are perfectly safe as long as a bunch of it doesn't get inside the scanner body. Have a paper towel on hand to soak up excess. Using flammable fluid for any type of watermarking does have a risk of fire, so be careful not to let any inside the scanner body. Once again, something like a paper towel will soak up any excess. If any does get inside the body, then unplug the scanner and wait for all of it to evaporate.
To perform wet scanning, first put fluid on the scanner's glass. Place your stamp face up in the puddle and get any air bubbles out from under it. Soak a piece of black paper (cut to somewhat larger than the stamp) and then add a few drops more to the stamp. Place the soaked black paper over the stamp and squeegee any air out. Finally, close the lid and get the scan as quickly as possible so the fluid doesn't dry out.
Another nifty thing about wet scanning is that you can leave the stamp in the scanner and let it dry completely. Then get another scan of it which will show all of the offset ink clearly. You could then use it as a mask in photoshop to remove all of the offset ink from your wet scan.http://www.stampsmarter.com/1847usa/Watermarks.htm
is great for learning what orientation they can appear in and how to use a template to see where they can show up on a stamp. Advanced photoshop users can use the images there to create a layer to place above the stamp instead of using a printout and a cardboard template.