Rust behind PU foam from waterline to decking

Bengy
I've followed and responded to a lot of these type of threads where someone appears on the forum with hopes for a project but the answers seem mostly negative, they become defensive and then disappear and you wonder what happened, do they feel beaten up? Do they look else where for more encouragement, do they give up?

I've rebuilt a very corroded steel sail boat of the same size as yours, I know the challenges and I very well know how long it takes and how much it costs. I also sandblast a few steel trawlers inside and out every year and assist with the repairs so I've a pretty good insight in to corrosion treatments.
The negativity here isn't to do with the feasibility of the project but what seems to be a miss-match between your expectations and what people who have done these kind of projects are telling you is the probable reality.
Bajansailor's advice to go over with a hammer is good. Ultrasound isn't easy to use and can give misleading results. The hammer however is cheap and effective. Go over the whole hull and deck with a hammer with firm blows on a tight grid. Thin areas sound different and have a lot of bounce or dent easily, you may hear the scale coming off on the inside. Mark those areas with a spray can and move on. Later you can test them with your meter or drill them. If you have thin plate in one or two places caused by say a leak then it is probably easily repaired but if the thin plate is wide spread caused by say multiple deck leaks through fastening holes then probably your project will take several years to finish and it would be much cheaper just to buy another, in this case you should consider scrapping the boat.