I would see how the structure looks when you remove the crud with something soft like a tooth brush/paint brush and WD40 spray.
On my 400 the sliders were stuck solid after 10 years of standing.
With patience I managed to release all four sliders undamaged using fresh petrol as a solvent, soaked overnight in a sealed tupperware type container. Three yielded in 24 hours, one took several days, refreshing the petrol every day.
