Peter,
You are really brave and I take my hat off to you.
When Beta Spyders were sold new, I had a friend who was the Sales Manager at a large London Lancia agent and he was more than a little unhappy at the lack of paint, never mind underseal or rustprooofing on the hidden parts of the Spyders (which were built by Zagato I seem to remember?). This dealer actually started a programme of trying to remedy some of this I seem to recall by painting the inside of the doors etc! And of course the steel used wasnt wonderful then either.
Having said that, the Beta Spyders were well regarded from the driving and aesthetic point of view so I am sure you will feel the effort has been worthwhile when you have finished your 'small' project.