[slotloco]

I found that tearing chunks out of a tyre occurs when too much pressure is applied.

Have to echo NF 
And nail varnish holds okay?  And doesn't cause any reaction in the tyre compound that would degrade them, or cause them to swell?
I've never trusted it anywhere near soft tyres, as most nail varnish has some pretty volatile solvents in it.
I am very particular about types of glue to use.
- but also, looking at that tyre - Mon dieu !!! what have you done to it. 
Hard to be sure without a partially side on shot, but it looks dreadfully deformed as if it was even fitted to the wrong size rim, or not glued in correct position, as the shoulder appears out of place.

I would say, we true ultra-grips on a weekly basis, across 15 plus guys, I haven't heard of anyone stuffing a tyre for ages.
A few guys are running Thunderslots now, and retaining the stock tyres, so they must be gluing and truing those okay.
( I haven't bought any Thunders myself to know the tyres,  preferring to retain my Slot.its, and prove they are just as quick in 65-80 sports.)

I PRESUME you are truing one tyre at a time, so you get a feel of the loading?  These drop arm lathes are rubbish when used to true two tyres at a time.
None of the brands, even the RSM when using the ball race 2nd arm, ever lower exactly parallel, and you never get equal loading on both tyres.
The claw mounts don't help with accurate/parallel seating of the two bushes anyway, it's just a limitation of the concept.

Usually I true on the slightly downward leaning side, as this naturally puts a slight camber onto the outside of the tyre, and provided better hook up[ and bite when you load the tyre up into the corner. Then you switch the axle around. You can mark the handle you turn on the lathe, to try and get both tyres the same diameter, but really, you also need to check them with a good set of digital calipers. 

As for grits, most people use finer than me, 600 is apparently quite common, but most guys in our club have 2 or 3 grits,
and they go progressively finer.
- You can put one grit on one end of the sanding plate, and another on the other end. 
We also mostly have a couple of plates, so we can swap out plates with different grits on them.


- A guy in our club makes up plates for all these brands of lathes, and also spare split pulleys,
and an advanced wide pulley which is better when doing alloy wheels where you can push the axle right through