So, in summary:
- With finishes such as nitro, each coating layer that is applied 'melts' into the previous coats. As such, you end up with a single, thick coating layer which can be sanded and buffed to a shine.
- The types of polyurethane products available to the DIY-er don't work the same. Here, each coat remains as an individual whisper-thin layer on top of the previous ones.
- Once you have built up enough layers of thickness to start flattening, then you will be sanding through multiple layers as you remove the dints, dips and dust buggies. Where you cut through to a previously flattened layer, that will show as matt. Where you cut through into a layer that wasn't previously flattened, it will show as gloss.
- exactly the same will happen if you use a buffing wheel. Here, the heat will tend to melt the varnish layers but you will still get the same contour line effect
So, with DIY polyurethane varnishes, I have found (but others may have other ways of doing it) that the trick is to:
1. Understand that the finished gloss is the final two or three coats you apply, unflattened and unbuffed (initially - see step 8 for more info)
2. Flatten the 'final' multi-layered coating, exactly as you have done, until it is perfectly smooth, dint and dust-bug free. I do this final stage with c 3000 grit emery, or something like 8000 micro-web, used wet.
3. Wipe clean of all sanding residue and let fully dry
4. Apply your 'final coat' of gloss
5. Let it dry fully. Any flattening matt marks showing through? If yes, then apply another gloss coat over the first one. It might need a third.
6. If there are other aberrations you are not happy with, no worry - just treat these two or three as extra pre-gloss coats. Simply repeat steps 2-5
7. Once you are happy with the dried gloss finish, STOP. Then leave it for at least 2 weeks to fully harden.
8. Once it's fully hardened you can use a good quality auto polish (I use Meguiar's Ultimate Compound) to hand buff it up using a clean non-lint cloth. You can repeat that hand buffing from time-to-time throughout the instrument's life - it may be a very thin final gloss coat, but it IS tough.
Don't use an auto powered buffer - this will generate too much heat and will soften that top gloss layer. If this happens, then you would need to to start again at step 2!
These are a couple of examples done this way: