Some of this kind of depends on how well the wood was sealed and what kind of varnish or resin was used to seal it. An additional factor would be how long the wood soaked in avgas. But generally speaking, Avgas isn't typically going to dissolve, solften or otherwise damage a good quality varnish or epoxy. If the Avgas just ran through the upper wing structure, I wouldn't worry much about damage, but of course doing a proper inspection is always a good choice.