MIMF

Hey guys. In an attempt to repair the bad paint on my Epiphone bass I sanded it down only to find out it was a plywood body. There I found an almost transparent layer between the body and the paint. It was probably half a millimetre thick. And it seemed it was very soft. It felt more like rubber rather than a solid finish/primer or whatever this thing was.
Initially, I thought the purpose of the layer is to smooth the body and hide the imperfections in the plywood, especially on the edges and between the layers. A friend of mine though suggested this layer had some sonic function - to reduce the resonance of the body and mitigate some reverberating (not sure this is the correct word). I've seen at least dozens of youtube videos and read at least as many how-tos about refinishing, but I have never seen anyone doing or mentioning anything about that.

So far I have covered the body with a body filler to hide the grains in the plywood and two layers of a primer. At this stage, I'm satisfied with how the guitar looks. (Until now) my planned next steps were 2 layers of spray paint and 2 layers of (2K) clear coat.

Any idea what this aforementioned layer could be? Do I actually need it? How and what to apply? If I actually need this can I do it at this stage - over the primer?

It's probably a sealer/primer that could be shellac based or something else. To test for shellac, apply a drop of alcohol, let it sit there for a minute or two, and then wipe it away. If you have a small shallow crater then it's likely shellac. You can apply just about any finish on top of shellac as it has almost universal compatibility. And compatibility is a real concern when applying different products on top of each other. It's not a good idea to do this unless you know for sure they are compatible or the two materials are recommended by a paint manufacturer.

The statement about sonic purposes for the layer is not probably the reason it was applied. Soft and thick finishes can dampen the resonance of a wood body, but only minimally. And dampening is not a desirable thing. Most all builders want the exact opposite.

Thank you, Barry, for your input. So I'm skipping it and for now forgetting about that. I'm pretty happy with the job I've done with the body filler so far so I'm proceeding with the paint now.

That coat was probably a vinyl sealer, very common for commercial nitro lacquer finishes.