1.    Am I understanding correctly that Boracare is the product to use in this application?
2.    After it is applied and let soak in, how would the surface be prepared for the shellac finish?

1) Boracare would be the best overall option. As explained in the treating furniture section of our powderpost beetle control article, it does the best job of penetrating the wood. This is key to killing off the larvae of this beetle. Jecta gel can be used too but overall, the Boracare is the best option and should be used whenever possible. Since the cupboard is unfinished, right now it would be a great time to treat.

2) It's best to let the treatment "cure" for at least 72 hours before applying any finish to the wood. Ideally, letting it stand for 7 days would be even better. This would insure the product had a chance to get deep down into the wood. It also insures the treatment would be dispersed well enough to do a good job of killing off the feeding young.