I dont want to try to tell anyone that their existing solution
to sticking valves is wrong, but I am unhappy with lapping.
I do not believe that lapping is ever necessary unless it is with new built or newly refurbished valves or valves that are too tight in the bore.
If an instrument has great valves that have worked fine without sticking for for 30 years or 50 years, what benefit does lapping bring.
Players love to lap valves the forum is full of advice to lap valves with lapping compound or toothpaste or vim or grinding paste or rouge.
The forum is also full of complaints that valves have lost compression in equal measure.
Removing metal by lapping from a valve that already fits the valve block perfectly, removes dirt and residues and valve metal that is needed to give good compression as well.
The valve has not grown thicker or fatter over time, it has grown fatter simply with dirt and residues that have accreted and stuck to the valve in use.
Techs remove dirt and residues chemically not by grinding the valve surface.
If I have a car with residues and detritus on the paint, I dont rub the paint down with glasspaper or grinding paste to remove the dirt. That removes the paint as well.
I soften the dirt and clean the paint.
If I have sticking valves I remove the dirt and residues that make them stick,
If a valve has been working fine for many years and suddenly starts to stick I clean it I dont shave the metal thinner.
That leads to loss of compression.
I have only one instrument with bad compression, and that has signs of past amateur lapping of the valves possibly to remove the dirt that made the valves stick.
I think the practice of lapping valves should be abandoned unless there is a very good reason to do it.