The peaks of the 500 stay larger and will make the ball grab harder than when you use the 1000/2000 over it, basically it takes more of the surface of the peak away. Keep in mind the 500/4000 surface is a very light 4000 over the top. The surfaces of ob nanos have read 600-900 on a surface topography machines. You are not getting anywhere near a 4000 surface. We just took my Soul from 500/4000 to 500/1000/2000 today and it added a bit of length and doesn't make as aggressive move in the mid. The underneath grit plays a huge role if you do it the way Storm tells you, which is about 5 secs on 4 sides with the 4000. If you use the top grip too long or with too much pressure than it will negate what I'm explaining. There are some good posts on bowling chat that show the progression in grits and underneath surface.
Here is a list I found on Bowling chat:
Rated from most aggressive to more length less aggressive.
From the surface changes I have played with i would call this accurate, didnt sound right to me at first either until I tried it.
360/500
500
500/4000
500/2000
500/1000
1000
500/1000/4000
1000/4000
500/1000/2000
1000/2000
2000
500/1000/2000/4000
1000/2000/4000
2000/4000
4000
Boy, Mr. Kowalski, you sure opened up a can of worms with that chart.
And people think I am exaggerating when I suggest there are differences between simple grits like, 1000, 2000, 3000 and 4000!!!
As a simple example, a ball finished with 500 grit, then 4000 grit will be rougher, in average, in general, compared to a ball finished with 500, 1000, 2000, then 4000 grit. And this is only step 1. Things remain simple if you use both grit pads fresh and with the same amount of time on each side of the ball.
So, a 500 grit pad + a 4000 grit pad will basically become (500 + 4000)/2 or 4500/2 = 2250 grit. Complications arise when you do the 500 grit pad for 45 seconds and the 4000 grit pad for 15 seconds. You're using the 4000 grit pad for 1/3 the time that you use the 500 grit pad.
Is it logical then to say that (500 + (1/3 x 4000)/2 = 1833/ or approx. 900?
Let's assume so for argument sake.
If you change all those grit sequences using a similar formula, then what those sequence becomes, on average, reflects the order posted, I suppose. But that takes grit levels to an almost absurd level of complexity with differences, truly tiny. With so many of today's bowlers having fairly firm ball speed, differences in ball reaction are less obvious. If you have slower ball speed, the differences you will see will be more apparent, to some degree..
In addition, there's the human factor involved. Are the pads fresh, are you using the same pressure, are you timing every application?