Foor deeper imperfections-
1-Start with 400 grit and be very careful of how much material you are removing. Get the imperfection close, and switch to 800 grit. Feather the surface outward from the imperfection. Any imperfection that really catches your finger nail (.040 or deeper. Lets say, deep enough that a toothpick would set down into the void more than half way....Hard to explain....) should be filled. The gel is usually thick enough to remove even deep scratches, however, you will see a low spot there, after the repair.
2-Only use 800 to even out the scratches of the 400.
3-Switch to 1200 grit and even out the scratches from 800 grit. This is where you can start using water. Water, is only used, so that you can see the scratches, and imperfections better. Other than that, the water has no purpose.
4-At this point, you can choose to hit it with 1500 grit (wet), or switch to medium cut compound. If you hit it with 1500 grit (or even 2000), you will save a little buffing time. Being as I have a good orbital buffer, I usually go to compound after the 1200 grit. If you used 1500 or 2000, even out the scratches from the 1200, and then switch to medium cut compound.
5-Once the surface looks very smooth, and you can only see the swirls/scratches under direct, bright light, switch to a fine cut compound. (proper lighting is VERY important). Continue with a fine compound until only fine swirl marks can be seen, when looking at the surface at a 45 degree angle, with light shining directly on the surface.
6-Swirl remover until mirror shine.
For Scuffs, marks, and light Scratches- Start at step 3.
I rarely ever wet sand. I have done it so much on cars, and boats, that I dont need to anymore. I will usually have a rag with wax/grease remover on it, and use it to swipe the surface after each step. This removes any chance of contamination, and also puts a gloss on the surface, so that I can see any leftover imperfections. It also saves myself from cleaning a mess after.