Not neccessarely. For example, guitar soundboards are made as a rule from spruce or cedar in 99%, and they always get darker through time due to sunlight. Thus, it depends on type of the wood.
The best way is to remove the damaged part and scrap (sand) off the old varnish (polyurethane). I would scrap it as much as possible but I would not go to the wood surface with the scraper or sanding paper in order not to hurt the wood. then I would apply (with a brush) only a thinner on a place where the crack is, in order to dissolve the old varnish - the crack will disappear after several actions if the remaining layer of varnish (polyurethane) will not be too large (under 0,2 mm). then you must put 1 or two layers of body varnish and after 24 hours a glossy layer. Of course air gun and compressor is demanded for this. But if you are not familiar with this, let the expert do it .... you have to know when and how to grind the body layer and after all the glossy layer must be polished to achieve shinny gloss.
But if you want to learn about this, then I suggest you to google "guitar varnish", "guitar polyurethane" or similar and you will find all answers
