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If the interior wood finish is in good shape, coated with actual varnish, and the varnish is not one of those 'chemically wrinkled' semi gloss varnishes, the easiest way to produce a most brilliant gloss finish is to 'hand rub' the varnish with 'rottenstone', a bit of water and a clean bare hand. If the surface is a bit rough you may want to start by 'flat sanding' the varnish with 1000 grit then 2000 grit wet and dry sandpaper. Rottenstone is a mineral thats just just slightly harder than 'talc' and is obtainable in most paint stores.
For a semi-gloss finish use rottenstone and oil and for a satin finish use fine pumice and oil ..... and a BARE clean hand.
You start by wetting your hand with water and sprinkling on the rottenstone, then rub the finish aggressively (until hot to touch), do this a few times then use very small amounts of rottenstone to do the final polish. This is somewhat labor intensive but will produce a finish that is *unbeleivable* ... and its the way that varinish is 'finished' in mega-yachts, private aircraft, and the 'very-high end' custom yachts etc. If the surfaces are large you can to the same process with 2000 grit wet&dry paper to flat sand, then use a highspeed autobodyshop polisher and 3M knobby foam polishing pad .... using 3M Finesse-It and 3M Perfect-it polishing compounds for the final polish. The finished surface will be be like a 'mirror'.
The heat generated by hand-rubbing or bodyshop polisher will 'develop' the irridescent glowing 'patina' of the wood cells beneath the varnish. The more you hand-rub the more the 'irridescent glow' from the wood cells you will get ... and the more 'brilliantly' glossy the varnish will become.
You can buff-out gelcoat to the same brilliant gloss in the very same way with a high speed autobody shop polisher and the 3M compounds.
For an even *more dazzling* surface finish you can use shellac ... applied with 'cotton wads' and methyl alcohol .... called 'french polishing' (do websearch). But this can be damaged by water so it may not be a good idea on a boat.
..... and on the EXTREME other end of the scale you can use "cetol".