We recently purchased a 100 year old home and one of the first projects I've decided to tackle is to fix up the basement bulkhead since its interior has become quite rusty. I'm guessing that this bulkhead is t least 30-40 years old. The exterior is in pretty good shape but the interior has degraded quite a bit. There's a lot of peeling paint and rust that I've already started to tackle with a metal brush. Once I have all the rust removed I plan to put down a coat of rust preventing primer followed by a coat or two of rust preventative paint.
One thing I'm not sure how to properly address is the threshold since part of is is so rusted out that it's got voids underneath part of it. When the bulkhead was installed the threshold was embedded into the concrete at the base.
Here's a picture of the threshold looking out from inside the stairwell:
And here's a closer view of the really damaged part:
My question is what would be the best way to repair this? Once I've finished getting rid of all the rust should I fill the voids in with something, and if so what would work best?