For those of you who have suffered from slipping valve lapping suction cups while trying to lap your valves,,
I found the fuel line slipped when applying enough pressure to do the job with a thumbnail to the valve head
I use a small tap wrench locked onto the tip of the valve stem. Spin with finger and thumb while applying pressure to the valve head with a finger of the other hand.
I finally finished the job today using every method known to mankind, including all the above suggestions. Lesson #101, don't use microfibre cloths to wipe paste from ports, the discarded lint is invisible to the naked eye , but thick enough to hold the valve off the seat, thank f#k for jewelers loupes