Before you start to sand the car you'll want to be sure that it's been guide coated, this will make it easier for you to get an ultra smooth finish. If you are trying to do a show finish on the car that your painting, you will want to sand it by hand with water running on it all the time.
This is the hard part, some people will want to use a machine to do this, this is just a word to the wise, you have a lot better control over a hand block. The best way to produce this type of high quality work is to have the best control over it that you can, often a machine will go through your primer. If your trying to produce show quality work this would mean priming that area again (i.e. more time spent, this is time that could be spent better doing other things).
The guide coat is the step right after the car has been primed, you should do this before you pull the masking, what this entails is misting a light coat of black paint over the primer so that you can see the low spots in your work, and no matter how good you are, you will have low spots. The idea behind this is to sand all the guide coat off with out going through to the metal on your car.
Now it's time to start the actual sanding of the car, you need to pay close attention to detail on this part of the paint job, the better you sand it, the better it will look. You usually start with 320 grit wet paper on a medium hard block, this grit is good for getting the guide coat smoothed out, there will most likely be some small low spots that will require either spot filler or more primer. This is one of those areas where you need to pay a little attention to detail, here you will need to look at the depth of the low spot and think about it, how low is it will primer alone fill it, or will it take spot filler and then primer.
Now that you've finished that part it's time to move on to the next grit of paper, you can move to 400 grit on a medium- hard sanding block from here, you don't want to move up to far because it can leave scratches form the previous grit of paper, so a word to the wise, don't get in a hurry and move up to far a once this will leave seeable scratches in your work. After you've sanded the whole car with the 400 grit wet paper then inspect it for bare metal and guide coat still there. The whole idea with sanding is to make the primer look the way that you want the paint to look, you should sand the primer until it has a smooth shiny finish on it, as if it were the paint on the car.
You need to have a vision of how you want it to look, the one thing that you need to know is, the better you want it to look, the more you will pay for materials. Just a word of caution cheap paint materials are just exactly that cheap and don't use them if you want a nice paint job.
You might save some money but you will not save the agony of a bad looking paint job. Think about this before you go and buy cheap primers and paints, do I love my car or is it just a vehicle to get me to work and back, if you love your car then don't use cheap materials.
From 400 grit move up to 600 grit wet paper, this is really as far as you need to go with the sanding. After you finish with the 600 grit do one final inspection of the work before cleaning it.
Well now it's time to clean the car, for this just use soap and water, just like washing a car normally. You should blow it dry though, this being the main difference between this and a regular wash job, be sure to blow all the water out of the little cracks in the car, like the cowl area, under the hood, between the doors and in the boot lid. Believe me this will blow water on your paint during the actual painting of the car, so be very through about this step.
If you miss some and it happens to get in your paint during the spraying process it will bubble the paint, the paint will look horrible so be sure to get all of the water out of the car first.
Now it's back to masking the car for the paint.
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After the car has been cleaned and dried, you can apply a high quality car care product like Farecla G10 or any other well respected brand onto the foam applicator pad. This product has the intent to remove: 
