It also takes a bit longer to dry. Smuding is worse than the new HP ink. Do not try to refill by just injecting some ink on top of the sponge. All difficulties experienced while refilling seem to originate from trying to let the ink soak in from above. If ink starts leaking, stop filling.
Some companies offer professional refill kits.
Usually these are a kind of accordian-like syringe with an injection tube of the correct length which contain enough ink for one refill. These refills are usually quite expensive, and the question remains if the ink they contain is any better than fountain pen ink. You could buy a professional kit once, and use the empty kit to refill the cartridges with fountain pen ink. This way you always inject the right amount of ink, at the right 'depth'. Fountain pen ink dries with a crust, clogging up the jets.