My uncle swore by thinning shellac with denatured alcohol for a 2-pound cut, but my test board showed major adhesion issues after a week. | Furniture Finishers - nhfree.com
Honestly, I've never had a problem with a 2-pound cut on oak if the prep is right. That adhesion issue sounds like a dirty surface or maybe old shellac. Fresh flakes are key, but the cut itself is fine. I stick with a 2-pound for a topcoat because it builds faster. A 1.5 just needs too many coats for my liking.