Most castor based fuels do not require an afterrun treatment, castor is a good sealing oil. On the other hand most synthetic oils are hygroscopic and with the nitro residu (acid/oxigen) rust will devellop. But even then if the engine is good stored rust isn't that huge issue.

On that pic is the result of driving in the wet or store it in a moisture environment like a garage/shelter.

When I have driven in the rain I first let the engine run idle with a rich setting. Then I drip some afterrun oil in the carburatot whil running idle, the engine will stay idle if you do it drop by drop. Then I open the throttle to a higher idle and flush some afterun into the carburator and the motor will stall. Now the engine is well treated to store without issues.