Badly rusted suspension fork
This is obviously (from the images) a suspension fork, and a very low-end one. Suspension forks are heavier than their rigid counterparts, but the trade-off is that they absorb shocks. These rust spots MUST MEAN the fork has long ago COMPLETELY LOST its ability to work properly as a suspension. As a result, you are carrying useless extra-weight, are not getting a suspension effect, are risking some worse failure along the way, and are risking some crash/injury.

The surface presenting the rust is the surface where sliding occurs (the "telescopes"). This surface must be perfectly smooth and shiny, and the rust shown necessarily means it is not, and never will be, as new again. Grinding will remove dust, but will not make the surface smooth nor prevent more rust to build-up afterwards.

My suggestion: sell the fork for metal recycling, and buy either a new suspension fork, or a rigid fork.

Hope this helps.