The sandpapers "gauge" are standarized with an scale... as far i remember the most fine are named "double cero" (or 00)... and also "water sandpaper" (because you need to drop some waterdrops on it when using it)
Thats the most fine that exist, you need to buy a couple of sheets of it
And additionally you should but 1 of the next gauge (not sure if is named "01")... and other of the next gauge (like "02")
You start with the 02.... then the 01... and finally the 00
And something i forgot to mention... after the sandpapers i continue polishing with a cloth + toothpaste, lol
The toothpaste is a nice/soft abrasive because it have fluor... and the cloth is nice to polish because the small fibers
The annoyance is the cloth is going to break several times, lol... (obviouslly, rubbing a metal piece repeatedly on top of a cloth destroys the cloth)
The way i do it is the same... i "grab" the cloth on top of the glass... then i rub the metal piece on the cloth repeatedlly (and this time with strenght) until the cloth breaks... then i replace the cloth and repeat until it breaks again... etc...
All this work requires time and insistence... most probably you are going to be moving your arm until it hurts
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Lapping the PS3 heatsinks is an annoyance, because are double
I mean... you need to do the lapping on 2 different surfaces (are 2 blocks of alluminium), one for CELL and one for RSX
One of them is sticked with glue to the heatsink "body", and the other is not (is a bit loose)
The point is... you need to separate them to do the lapping individually... and this is tricky and hard to explain

Im goign to show you a photo of how is made the heatsink of a CECH-25xx
In this phtoso can be seent he 2 alluminum blocks, the block at botton (with a bump at his right for the copper pipe) is glued to the heatsink body (and is imposible to unglue it, btw)
https://www.psdevwiki.com/ps3/File:CECH-25xx_heatsink_(bottom_view).jpg
When you look at it from this perspective you are going to notice the other alluminum block have several small bumps in between the fins.... are working as some kind of "retention bumps"
This block is not glued to the heatsink "body", the only thing that keeps it in place are that "retention bumps"
If you break that retentions you can take the alluminium block out of the heatsink
https://www.psdevwiki.com/ps3/File:CECH-25xx_heatsink_(back_view).jpg