I lapped my Sandy on a piece of finish plywood. It produced an excellent finish. Right now, I'm lapping my new Thermaltake Northbridge cooler. I'm using a plate of glass that covers a picture.The glass is not all that smooth.
This is my 2nd lapping job, and I spend a lot of time working with the 400 grit first. I found that the bottom of this cooler was , indeed, quite concave. The 400 grit will help to smooth it out more quickly than the higher grits. I'll use 800 and then 1000 to finish. Hopefully, the photos will come out clear
One thing that you will notice is that if the HSF surface is not smooth, it will tend to "catch" on the paper as you move it around, so go slow to begin. I used water as a lubricant with the 3700+. Not too happy with how that worked. This time , I'm lubricating with 91% Isopropyl Alchohol.