[xtremech]

I agree. When you do lap them lightly, use nothing more than 400 grit. You may have to hunt a little to find it but it will not lie to you, 280 grit will.
MSC or GRAINGER has everything.
REMEMBER, If you have to lap more than once....someone did not do a quality job on the valve job. Typically, all Porsche valve seats are very hard.
It takes ample time and effort to get a concentric seat.
Spend the time with some brake cleaner and a lint free white rag after lapping to remove any residue on the seats and valves.
Good Luck