Since these are toyboxes for the kids, I use a wipe-on poly, which is very durable and safe. When I do fine furniture, I usually do hand-rubbed oil - which is a hell of a lot of work - so this is much quicker and more relaxing.
The background on the carvings naturally comes out a bit darker as the finish is not as smooth as the surrounding field. The surrounding box is machine sanded to 320 grit, but I leave the background to the carving a bit rougher. The finish penetrates the rougher surface much deeper and results in a darker appearance. When you get into higher end stuff, there is a real art to managing the surfaces for the purpose of getting the desired finish effect.
Here is a finished box which shows this a bit better. Also notice the end-grain portion of the box joints is also darker than the face grain - and for the same reason. Finish also penetrates end grain more easily than face grain. If desired, you can actually get the end grain to look the same as the face grain by sanding the end grain to a higher grit. In this case, I like the contrast.
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