Rubbing alcohol removes most "permanent" markers form a solid surface. But won't remove the paint marking on the IC. So anyone that knows this will still get to see what they are. Ramussons has it right.
I'm no expert at this, but every case of the manufacturer hiding chip details I have seen has been what the guys said above, rubbing the top of the chip clean with some sort of sandpaper or grinder.
Hiding the chip part number is useless because if it is profitable somebody will reverse engineer the product. Low selling price does the trick, or commanding the market upon release helps.
It depends on what your purpose is.
Apparently your purpose is profitability. My purpose is different, and sanding the numbers off has worked perfectly for decades.