Well, this got slightly out of hand, but in the end I decided to try and separate the deck and hull, since I wanted to add Maroskes and these would have been very hard to do otherwise, plus I used a sheer clamp instead of inside glass on the seam, and there was some more water penetration in one place between the sheer clamp and the hull.

To my great surprise, separating the hull and deck was actually quite easy: I used a very thin paint scraper and a heat gun (set to 500 C) to heat the scraper, not the seam and then slid the hot scraper into the seam (no exterior glass, since I had already removed it.

Result was two halves of my trusty Petrel, ready for refurbishing!