Lapping works good, just more work, and prolly unnecessary if the valves aren't leaking already. If the valves are leaking, it's usually due to mis-alignment caused by worn guides.

Can you lap valves too much?

Although some seats may need two or three times of lapping, it is not necessary or good to over-lap the valves. It is unlikely, but possible, to remove too much, and doing so will ruin the valve seat. If you are unable to achieve a consistent grey ring, the seats are probably worn out and will require cutting.

Is valve lapping effective?

The current methods used in most automobile maintenance businesses for valve lapping process are not effective and consume a lot of working hours. 'Valve lapping Machine for Internal Combustion Engines' is a machine designed to overcome these problems by minimizing the human involvement in the process.