First, before I bought the boat, I was told that the water in the forward bilge was rain water.... I agreed to make the purchase contingent on hull testing called an Audio Guage. This was performed along with a survey. The results showed one area of particular concern, but the rest of the hull only had "minor" deviation from what was anticipated as original.... the hull soundings were done in a systematic pattern and the company gave me a chart of the boat. Problem with this is that specific areas, "not apparently leaking" could be passed over.
In transporting the boat after the purchase, the hauler loaded her stern forward and removed the canvas top over the deck. Then the worst thing happened.... the weather was horrible and I mean RAIN!!! for over 200 miles the bilges filled with water....
What happened next was horriflying as well as welcome in that the boat had always been in a fresh water environment and I was converting her to saltwater... the second day in the yard, this water had penetrated the paint in the bilge and the rust below began to bloom... It was ugly to see this red water... red from rust.... for two weeks, I chipped, scraped and brushed rust... and removed all of the junk... then I found a Needle Scaler.... that cut the work down...
Once this was complete I took OSPHO to the metal. It is great stuff. I am still waiting on the welding, but from here I will finish the cleaning and then prime and paint the entire engine room...
If I were rich, I would remove the engines, tanks and eveything else and sandblast the entire compartment, but then I'm nowhere near rich enough for that...
Basically the areas you need to really inspect are under the tanks, batteries or any other area that may have been basically untouched since she was built....
I know you hate the idea of plating, but think about it... it's Steel and you're probably not looking to race anyone... so a little extra metal, isn't really all that bad.... just make sure they seal it well.
Sandblasting??? the only real way to know!!!