Charles and Queen Consort Camilla had the important duty of signing the visitors' book Friday in Llandaff Cathedral, and while the royal attendees gotta be on edge from the last time he whipped out a pen ... this time, King Charles got the job done with his own utensil.

Pretty hilarious, considering the inky situations he's had all week.

You'll recall, things got really sticky on Tuesday when the King attempted to hand sign a guest book at the royal residence of Hillsborough Castle in Northern Ireland ... and the pen leaked all over his hand.

KC3 threw a tantrum and stormed off muttering "this bloody thing" ... leaving Queen Camilla to sign for him.

That leak came on the heels of another meltdown ... days before he came across a pen box on his desk while trying to sign a proclamation. He was not here for the box being in his way ... and motioned for an aide to quickly take it away. It's good to be the King, as they say.

Well, he's learned his lesson, and we gotta say ... Friday's signing was smooth and calm. No leaking ink, sweat or tears!