Prop flange rust?

The only way to remove rust is via mechanical means, like Roger’s Dremel spinning wire wheel (watch out for flying wires though, they hurt!) suggestion Even then some corrosion may not be reachable via mechanical means without removing too much of the substrate. There are rust conversion coatings (I’ve used an Eastwood product with success) that will flow into every pit and crevice and encapsulate existing rust and prevent further corrosion. Cures as a shiny black coating. Requires a top coat of paint after application. If you decide to go that route protect EVERYTHING else around the flange as the conversion liquid is messy, flows rapidly, and removes any paint it touches. 

From RW

".....(waxy properties)....."

From Rotax SI

"* or equivalent # long-term, waxy film rust inhibitor"

Tectyl 506 spray can AU $20,  can be got from most automotive outlets, some paint shops, etc

Originally developed as a corrosion inhibitor for European cars (salty roads) now has many other uses in marine & agriculture. Great stuff - I love it.

Can be purchased in pack sizes from  400gms - 4L- 20L

https://www.valvolineglobal.com/en-au/tectyl-506-heavy-duty-rust-preventative/