Quoted from Mrjamma:Do not "sand" edge connectors. It ruins them by removing the tin plating and coating them with solder is not a long term repair. It will give you more trouble in the future.
You can create a bigger problem than you started with.
How would you fix a sys1 board with heavy corrosion/rust on the pins then? Sometimes sanding and re-tinning is the only option besides buying a new board.