The hatch going down to the engine room had rusted a lot around the lip where water had been allowed to collect. There was also an overhang where the steel could not be proper sandblasted when it was built first.
Here is a diagram of where the rust was at its worst and a photo of the hatch before I started work on it:
First I took the hatch off and knocked out as much rust as I could with the hammer drill.
I then took the angle grinder and cut way the lip all around the edge. After doing this I found that there was only one side where the wall of the lip had rusted through completely. This was a big relief as I was dreading having to replace the whole valley all the way around the hatch.
I cut out the very weak metal and cleaned down the rest of it:
I then welded in a new piece of off steel and grinded everything flush.
When this was done I went around with a wire brush to clean down the entire valley and painted it with Epoxy 2 Pack.