Grandad's cartridge method works. I've been using it for months now, and its quick and clean.
The problem is the prolonged drying time. Heating the cartridges with a microwave oven is a bit dangerous and definitely not convenient. Here is a simpler method:
I use a small piece of tissue (kitchen paper is probably the best). Place the tissue on the exit port and hold it with your finger until you feel your fingertip getting wet. Remove your finger and let the cartridge upside down.
As long as the cartridge contains water, the tissue will stuck on the exit port and water will start spreading on the paper. The water will be evaporating from a much larger area leading to a very quick drying of the entire cartridge.
I've just used this method on 9 cartridges and it works with good results. Hope it helps
The problem is the prolonged drying time. Heating the cartridges with a microwave oven is a bit dangerous and definitely not convenient. Here is a simpler method:

I use a small piece of tissue (kitchen paper is probably the best). Place the tissue on the exit port and hold it with your finger until you feel your fingertip getting wet. Remove your finger and let the cartridge upside down.
As long as the cartridge contains water, the tissue will stuck on the exit port and water will start spreading on the paper. The water will be evaporating from a much larger area leading to a very quick drying of the entire cartridge.
I've just used this method on 9 cartridges and it works with good results. Hope it helps