Good advice above. Yes, external solenoids do get pitted contacts, and make poor connection. When they make a poor connection, they arc more, and pit more. If you are able to take it apart and clean up the pitting, great. Though some solenoids do not lend themselves to disassembly - replace. Starter motors with internal solenoids are not quite so simple, though very disassembleable.
Note that in some [older] installations, where the chassis system as 12 volts, the starter solenoid was a 6 volt, so it slams in hard to make good contact. Ideally, it was not energized long, so it would de-energze before it got too hot. I'm not saying that you should change from 12V to 6V, but some farm vehicles were made that way.
Also note that an engaged starter motor can pull a weak batter voltage way down. Indeed, if the solenoid can handle it, a 12V battery with one shorted cell, might still start the engine (ask me how I know!). So check your voltage again with the starter load on the battery. If you see it down around 9 volts, it's time for a new battery. Keep a spare solenoid in stock in any case!
