Flushing chemicals as used in central heating will clear out sludge but they will also expose any holes. If the matrix has corroded patches** its best to swap for a new one.
**Green for copper grey for aluminium
But it can still be worth doing a chemical flush, though not when its likely to freeze overnight or if the car will be standing for long periods. It needs to be pumped around hot so the crud can dissolve and then be FULLY flushed out and refilled with proper coolant.