With that out of the way, Anyone else who doesn't know that electricity, fluids, and metalic dust don't mix, sorry, but you had better pack it in too. for now anyhow. We'll be making a lot of conductive slury and dust, and it WILL fry your electronics if they get it on them. PUBLIC SERVICE MESSEGE ENDS. If you fry something, or someone from here on out, it's your own fault.
Set up your lapping surface. Fill your water container. Turn on some tunes.
Examine what you want lapped. If it's a CPU, GPU or simmilar, tape it up to protect the stuff you DON'T want lapped/wet/etc. If it's a heat sink, take the fan of, and we're set to start.
Choose a grit of paper, based on the beginning roughness of the heatsink, or the out of flat of the CPU.
A very rough heatsink might require 180 grit to start. My AMD64 only called for 600 grit to start. It's a case by case descision. Use your head.
Tape the paper to the plate, and wet throughly. Place the object to be lapped onto the paper, and apply gentle even pressure on the object. Begin moving your hands in a gentle circular movment. Don't press too hard, and let the abrasive do it's job. If it begins to dry out, just add some more water. it acts as both a lubricant, to aid your even movements, but also flushes away particulates, preventing unwanted scratching from built-up crap, in the abrasive paper.
Every three to four minutes clean off the heatsink and examine it for an evenly abraded surface. While you're at it, brush the built up crap off the abrasive paper, and re-wet. Once the surface is uniform, it'stime to jump up to the next grit. Once you have finished with 600 grit, you can stop. with some quality thermal paste, this is the best finish you'll need for most "minor" chips. Anything that comes in a ceramic or plastic chip package has a rougher surfage anyhow, and the thermal paste will bridge the gaps satisfactorally.
For CPU's and GPU's, or any thing else where BOTH surfaces will be lapped, this is where the fun begins. Tape down your 800-1000 Grit, and get back to work. It may not seem like you're doing much at this point, but it will pay off. when you've worked your way through 1200 grit, stop and clean everything well. Your paper, lapped surfaces, Plate, Everything. Change your water for fresh water. The dust particles that fell into it (if the top was open) are bigger that the particles of grit we'll be working with now. You should have an even, dull surface on your work. It should take a magnifying glass to see ANY scratches in your work. For almost any application, this is good enough. It is flat, smooth to the touch, and better than almost any factory finish.
This is for the rest of us. Get that 1600-2000 grits ready to go. When you're done with these grits, spend some extra time with the 2000 Grit, even after you have that final finish. As the 2000 grit is used to achieve a dull miror finish, it wears a bit, becoming an even finer grade, nearer to 2200-2400 grit. working in the worn areas SHOULD result in an high polish finish. You should be able to recognize clear newspaper text reflected in itat least.
Don't get too hung up on achieving a "mirror polish". While trying to get it THAT polished, you may actually get a WORSE finish, microscopically. On that level, the waves of metal begin to bend over, appearing smoother, but creating pockets of air, trapped between the layers of metal. better to fill this in with thermal paste than sanding grit and air.
Finish by cleaning with some Iso Alcohol and a lint free cloth (NO! Not facial tissue!) Now, admire your work. OK, all better. now clean it again! I KNOW you ran your finger over the sexy smoothness. Make sure to get all the bad Oil and fingerprints and such off it. Apply your thermal paste, and get ready to run your stress tests.
As a special note for copper lapping, after your finished lapping, get the thermal paste on, and the object in place as soon as possible. A slight oxide layer will form in most coppers, that will sap 5-10% of a degree from your final tempature reduction. Aluminium also forms this layer, but MUCH faster. Parts of a second, versus minutes. Sorry, it can't be avoided. But, in aluminium, that THIN layer does serve as a barrier to any FURTHER oxidation, protecting the majority of your hard work. It is also the layer where Annodizing is done, so if you want a lapped heatspreader for your Dimms that is also colorful, lap BEFORE anodizing.