is there a transparent substance . . .

I can use to fill a carved piece of wood up to a flat finish? Most guitars . . . Fender are encased in poly urethane. Will this shrink and have slight but still noticable depressions? I want to carve something into my guitar and then fill it so that it has a completely flat finish without any minor depressions?

I'd have asked a luthier but there's no luthier or guitar builder groups. Better to ask materials scientists experts.

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Find a cheap/broken/replacable guitar to experiment with. And understand that the following advice is only worth what you paid for it.

The best approach would be to fill the depression by making many coatings with each coating being a very thin layer. This approach will take a good deal of time, but it can be done. The Japanese make beautiful coated pieces that have up to 1000 layers on them. So adopt a Zen master approach and proceed. Make sure each layer is thoroughly dried and cured before adding the next layer, or the whole thing may look cloudly. Be sure to following the recommended safety procedures for applying the coating as some of them could contain solvents (good ventilation, proper gloves, no ignition sources nearby, ...)

Two options to consider. The first is to use the same coating that is already on the guitar. The guitar manufacturer might be willing identify what the material is.The second option is to use a totally different material. This will create a difference in refractive index between the two coatings which will cause that interface to stand out more than if you used the first option. The greater the differences in refractive index, the greater the impact.

As the depression is finally filled, you will not be able to match the exact height with the existing body, so you will need to sand/polish/buff the area to accomplish this.

John Aspen Research, -

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"Turning Questions into Answers"

Opinions expressed herein are my own and may not represent those of my employer.

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Hobby shops carry two part clear resin systems designed to pour into molds and cure bubble free. They do not shrink on curing. One of these will work for you.

Resins carried by solvents will suffer from the shrinkage you worry about because of the loss of solvent volume during hardening.

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