This is a cut and paste from a previous thread, but should get you in the right direction on polishing. Also, make sure you use quality sand paper (3m). It will last ten times as long.
Polishing acrylic 101
I'm going give everyone a a few tips and hopefully show a few of you guys how to polish out some of the old acrylic tanks we all have sitting around. Maybe even give you the confidence to remove that big scratch in our main display tank. I would first like to say that I'm not an acrylic expert like Lee. I don't work in acrylic everyday, nor have I ever. I gained my knowledge from owning a detail shop in Lake Havasu City about ten years ago, and master my skills by polishing boats and cars. I realized when I got in aquariums/reefs a little over a year and half ago that all the same principles to polishing paint transfer over to acrylic for the most part.
Lets get started
Step #1 WET SANDING
There are a few different ways to do this. I have found the most efficient method is a D/A sander with a 1000 grit sand paper. I will basically wet sand the entire inside of the tank with the D/A and 1000 grit. If you dont have a D/A sander. You can use a rubber sanding block. Just make sure you dont use just your fingers behind the sand paper. If you do, this can lead to grooves in the surface your sanding & you will not be happy camper. I try to stay away from the corners if possible since its hard to get a polisher in the corner. I'll cover that a little more in the future. If there is any deep scrathes (one that you can feel if you slide your finger nail across it), I'll take a sharpie and draw a line in the scratch. This will help you know when the scratch is gone. Basically the sharpie high light the scratch. The scratch/shapie mark will slowly disappear/fade as you wet sand the surrounding area down to deepth of the scratch. If scratch's are too deep, sometimes they need to be left alone, or polished out only slightly. The only reason I would ever leave a scratch, is if the scratch is so deep that it would create a visual distortion if it was sanded out. This is very rare instance. Let me go back a little. The second reason I use the sharpie is because once you start wet sanding, the water will turn into a white milky solution, and the acrylic will turn into a white haze. The scratch's will get lost in the haze & only seem to re-appear once you start to polish. Then you have to go back and wet sand that spot again. This creates the one step forward, two steps back effect.
If you dont have a D/A sander, I would highly recommend buying one. You can pick up a cheapo one at Harbor Frieght for about 30 bucks. Your arms will thank you for it & you'll save yourself a few hours.
I know everyone has thier own opoins on what grits to use. I dont really recommend using anything more then 1000 grit. If you use a 600 or lower you'll blow right through the scratch's, but you will also be creating additional steps by having to remove scratch's from the 600. I use 1000 then 1500. I don't really see the need for anything higher since the high speed buffer will remove the scratchs from the 1500 with ease.
So now you have wet sanded out all the scratchs with the 1000 and 1500 grit. Your tank should look like a smooth white haze that's difficult to see through. All the scratch's should be gone. Now your ready for the next step.
Step #2 POLISHING
There are a few tools that most of us already have, and few you may need to purchase. They are all pretty reasonable prices, unless your buying MAC or Snap On.
#1- cordless drill
#2- varible speed polisher with a velcro backing plate
#3- wool/foam polishing pads
#4- Mothers Power ball or cone
#5- polishing compound (NOVUS works well)
I start by putting a wool pad on my 7" varible speed polisher. I start with a wool pad over foam since wool has the most cutting action. I then put a little compund on the pad, and smear it in a little. I smear the compound to prevent it from slinging off the pad once the buffer starts spinning. Start with the buffer on the slowest speed. You can increase the speed a little once you get going. It does take practice to get comfortable with the polisher. I'll go over the larger surfaces with the larger polisher. The large polisher will give you the best results. It works pretty fast compared to the other methods. Just make sure you keep the polisher on a low speed, keep the polisher moving at all time to avoid heat/compund build up. Make sure you keep the pad tiled at a slight angle. The rest you will just have to figure out by trial and error. I also keep a squirt bottle filled with water, I will mist the surface I'm working every few minutes to get a little more work time out of the buffing compound, and keep the tempature down.
You want to make sure that you don't get too much heat build up, the heat can cause crazing.
Once your done polishing. You'll find some spots you missed, or some scratchs you didn't notice. Just go back and sand them out and polish that spot again.
I like to use the Mothers power cone and my cordless drill get into area's where my polisher could not reach. You'll find that the corners are a little difficult to get into. The cone gets in there pretty good, but the results are a little less than that of the polisher. Once your done polishing, you need to was the tank out & get rid of the compounds that were slung around.
All in all, polishing out acrylic is not rocket science. It is labor intesive and takes some time. You'll get better with time, but can save yourself a ton of cash. I Think ATM charges like $75 an hour. If anything, go slow, take your time and you'll be happy with the result