In the US most notaries will not allow you to sign official documents with a fountain pen- it seems to be part of the training, and many realtors and attorneys are notaries for convenience. As great as they are to write with, many FP inks cannot withstand tampering, so they don't allow any FP inks because it's hard to track which of the thousands of brand/color combos are safe. A cheapo ballpoint is better for official documents- although in this era I'd trust a cryptographic signature more than a handwritten one.