Sold Solo, and Cima mistblowers up to 300 gallons a few years ago.
Tanks, were mostly poly so not as critical as metal.
Simply drained, flushed entire system, left tanks, filters and valves open and let air dry.
Pumps all drained, flushed, then filled both pump and attached valves with anti freeze. Tried to agitae to get everyting coated.
When reactivating....cleaned, flushed, cleaned, flushed, flushed.
Today, I would do one of two methods after exploring both products:
1. Clean, flush, forced air dry, then install a solid vaporizing rust inhibitor. System is kept closed for internal coating.
2. Same as above, but would install a long term storage aerosol vaporizer. Come in spray cans, used in treating engines.
Gets rid of using messy, and poisoness antifreeze, besides would save time.
Both would work, just don't know which would be simpler. Cost isn't a major factor as the results of idamage thru improper storage are worse.