Carpenter bees do not eat wood but rather drill holes inorder to lay eggs. This year has been a banner year for carptenter bee activity. There is no carptenter bee repellent per se. You can screen in times of high activity. If you have bees currently attacking the doors, that is somewhat surprising in that they usually like different kinds of surfaces. If they are not attacking, such doors would not be their favorite. One possible solution would be to use a high impact polyurethane rather than varnish which will give a stronger finish.