To service the blakes sea valves is not rocket science.
You usually can get engine
grinding paste from any good motorist store, try and get 'fine' or 'Medium' grade.
the valve a bit, smear both the valve 'inside' and the 'insert' part and grind them 'TOGETHER', not apart
, using a rotary back & forward motion and up and down from time to time.
I spend about 10 min with the medium grade then about 15 min with the fine grade of paste and grind them until they are smooth (not shiny)
Make sure the valve body internally and the insert part are absolutely clean before assembly.
A simple test is with a pensil, draw three vertical lines (top to bottom) down the outside of the insert part, then grind, all three of the pensil lines should disappear, any part showing says it is not fully ground.
The trick to assemble them is with the best thick quality grease you can buy, Blakes make thier own grease for the valves which here in the UK it is £12 a tin,
BUT it will last you and the valve.
Do not tighten too tight on assembly, the handle should turn easily, if any sign of drips later just slightly tighten a bit more, as a guide I can turn mine with one finger.
Hope this helps.