When clear wraps are done successfully the threads and finish
should look like a clear band without any visible thread. There is a never ending quest on this forum to conquer the secrets of clear wraps - often with the results not meeting expectations. There is
a lot of science happening. Bear with me.
Rod wrapping materials are colored, although the colors are not easy to perceive (as
merkexpress showed above). In addition, rod wrapping materials have a physical property called
refractive index or RI. The RI defines how much light bends when it enters a material. Here's the important part.
If your thread and wrap finish are the same color
and have the same RI, the wrapping threads will be invisible.
How does an ever questing rod builder get to clear wraps? Practice, test, practice, and test. Take notes. Try different materials. Test some more. Once you have a system, stick with it. Don't wing it. Test any material substitution or new batch of thread/rod finish to be sure.
So what could make clear wraps not so wonderfully clear?
- Color. Truly colorless thread and finish are hard to find. The underlying rod blank brings another color to the party. Perceived color changes with lighting*. Evaluate your tests outside under the conditions you will use your rod.
- RI dispersion. Dispersion measures RI changes with respect to the light color shining on the material. In other words, the RI changes as the light changes. Test wraps look different under workshop lights than in bright sunlight or on a cloudy day. Again, evaluate your tests outside.
- Uncured finish or impurities. Spar varnish color and RI changes as the solvent evaporates and the oils cure. Epoxies change RI during cure (and turn yellow if poorly mixed). Don't evaluate your tests immediately - let them cure for a week or three.
- The final color and RI of the finish. Varnish and epoxy makers don't publish their color or RI values. In addition, the RI and inherent color differ from company to company. Traditional spar varnish has a yellow cast as does polyurethane spar, but they have different RIs. Different epoxies have different RIs. Test different types and brands of rod finishes.
- The color and RI of the thread. Thread makers also don't publish their RI values** or the secret sauce in their colors. White nylon and white silk do not have the same RI. Different white threads from the same company are different colors. Don't change threads without more testing.
Most important, be reasonable with expectations. There are many factors to juggle. Your camera shows things your eyes can't see*. Macro-scale photos reveal color and RI differences that are imperceptible when holding a rod at arm's length. Clear wraps are tough to achieve.
Tom
* Cameras see light much differently than our trickster vision. A sheet of white paper under a typical room light might photograph as orange. The same paper outside on a cloudy day might photograph as blue. Our brains know that paper is white, so we will always see white.
** Thread filaments are also birefringent, meaning the RI changes depending on the direction the light hits the thread. This is a minor effect compared to dispersion and color. Bulk RI values for nylon and silk are published, but the birefringence increases as the materials are pulled into filaments and those filaments are spun into threads.