Help analyze personal inspection findings (4 of 5) - coolant deposits

[pillars]

Topic 4 - how much deposits (rust?) is typical in a coolant overflow tank?

Note that this engine has not been run in months, so deposits would be settled. The picture on the left if just dipping a stick in the coolant overflow tank. The one on the right is after scrapping the bottom of the tank. There is obviously some rust deposits in the system. Is this a typical amount of rust or indicative of a larger problem. Should other tests/checks be performed during sea trial to look for the root cause or is this a matter of flushing the cooling system?
Engine is 2005 volvo penta 29 hp with 3700 hours. Starboard, pictured, had more than port.