Valve Lapping and Valve Clearance

Does anyone have any experience with improving valve sealing by lapping valves? I know this is kind of an old art, and the FSM calls for doing it after the valves are (ground - complete valve job), but I've seen a lot of internet videos of people doing this to "improve" valve sealing. I am not getting any increased sealing as a result of the valve lapping.

I have one set of heads (from a 150K miles VG33E), that aren't holding up to my fluid test. I flip the heads upside down and filled them with diesel fuel (I used water in the past, but if I forget and leave them with water for a couple days, the valves begin to rust), and overnight, the fuel all leaks out. I tried lapping the valves, with coarse and fine grit compound, but I don't see any improvement in the sealing (verified by repeating the fluid test).

The engine had 155-160psi compression before I broke it all down (replace the HG and bearings, and rings for a refresh), an I thought that was "decent", but I'd like to improve valve sealing a bit before putting it all back together. Another set of heads I have from a 50K-65K miles JDM engine I bought holds the fluid overnight just fine. I even tested them with blowing compressed air into the intake and exhaust ports. The 150K miles heads blow bubbles badly. The 50K-65K mile heads bubble just a tiny bit (exhaust are a tad worse).

I was hoping to find a low cost method for refreshing the valve sealing, without a full blown valve job. Anyone have any ideas?

Also, if I do have to grind the valves, do the back of them valve stems also have to be cut, to keep the valves in spec for clearance? How does one measure clearance? Can it be adjusted? Do the hydraulic followers take automatically take up the slack on the ground valve faces and seats?

Thanks!!!