Linseed oil as violin polish
My violin's varnish is new and fairly thick, and yet lacks any gloss.
I found (by applying to a small area!) that boiled linseed oil dries to a nice shine. It's a bit sticky, though, and rubs off.
Any better alternatives? French polishing how-to?
Use the search box. Lots of threads already on DIY violin polishing. Not generally recommended, but that doesn't seem to stop you.
Linseed oil never dries and if there are any loose edges or open cracks, after contaminating them with linseed oil, they will never glue shut/closed.
So long as the music shines I would leave the instrument alone.
Personally I like a "satin" appearance.
"Linseed oil never dries" is not true. I hardens by oxidation. The process is normally very slow, but boiled linseed oil as the OP mentions hardens much more rapidly. It is used as surface treatment for wooden floors, furniture etc. "Boiled linseed oil" these days is most often not boiled but has chemicals added to speed up the hardening process.
Linseed oil will remain liquid for a time, during which it will act like a primitive air filter, collecting dust and gunk. At some point, it will harden, incorporating the collected grime into a varnish-like coating, which can be very difficult to remove. Not recommended.
If your varnish is new, let it cure before attempting anything. Do you know if it is oil-based (my guess) or spirit-based? If you don't know it's one more reason to leave it alone! Oil-based varnish doesn't dry through exposure to air, but rather to light, and under normal conditions can take months, if not years to cure. The curing will affect sound in a significant way. Luthiers use UV light boxes to speed up the process... to a point, but it still takes quite a long time even after that to fully cure, arguably decades. You should never do anything to varnish if you don't know what it is to begin with, and chances are you don't have the slightest idea what your varnish composition really is, unless the maker told you.
I agree with Bo.
Roger - "Oil-based varnish... can take months, if not years to cure."