The four-platform station sits on a pier, supported above the tidal waters of Portsmouth Harbour by a 19-span viaduct.
A comprehensive scaffolding structure is being installed over the water to provide access to the underside of the 250m-long station, via a disused track.
The £6.2m project involves strengthening and repairing the main supporting sections, along with surface preparation and re-painting.
Osborne project manager Martin Parsons said: “This is a technically challenging project, not only are we working on scaffolding over tidal waters, the tide itself gives us a limited working window. Collaborative working with both our customers and our supply chain partners is vital to ensure this project is carried out safely and without unnecessary disruption to the public.”