If you're finishing your own guitar without access to a paint shop then chances are, no matter how good a spray job you do, you'll have some orange peel texture to deal with. But fear not, it can be transformed into a perfect mirror finish with a lot of patience and a little effort.
The guitar body above had a fairly significant amount of orange peel when I sprayed it but any amount of texture can be dealt with using this method. Firstly I'll say that this is only my method, others will have their own ways of doing things so read them all and choose which you're most confident with. So this is how I do it. Firstly and by far the most important is that the paint has to be completely dry before you start sanding and buffing. For me this means that if I have a nitro finish guitar I will leave it about 5 weeks in my house (at 25-30 deg C) before sanding and buffing. If I have a poly finish guitar I will leave it to dry for 2 weeks. This is the most tempting part to want to rush but the easiest to get right.
The process I use is to wet sand from 2000, 3000, 5000 and then use cutting compounds and polish. If I have a lot of orange peel, I'll start wet sanding at 1000 grit. All sanding is done by hand with a cork sanding block.
The fist step in the sanding process is to level out the whole finish, this is where the 1000 or 2000 grit is used. When wet sanding I use a squirty bottle that atomises water and most importantly a teaspoon (or one squeeze) of dishwasher soap. This makes the whole sanding process smooth and prevents deep scratches from occurring. Wet the sandpaper and a few squirts on the guitar body and sand the surface until you feel that the surface is smooth underneath the block. You'll feel the roughness of the orange peel initially and then this will go. So go over the whole surface once, then wipe down (the guitar surface and your sandpaper) to see how much is left to sand.
When you look across the surface you'll notice that there are shiny spots you have not sanded yet all across the surface. This is where you'll need to sand down to before moving on. You're only done on the first grit at the point the whole surface is matt and smooth with no low spots.
As you move higher up the grits the sanding becomes quicker and a simple way to ensure you're removing all the scratches from the coarser grits is to sand in a slightly different direction with each grit. This way you'll see any old scratches that still remain from the last grit. Once they're all gone, you can move on, but be sure to thoroughly clean the surface before starting the next grit because you don't want any 1000 grit still on the surface once you've moved to 2000. It usually takes me 3 to 4 passes per grit to get the surface perfect before moving on, wiping down and re-wetting the surface with each pass.
I never sand higher than 5000 grit because cutting compounds are perfectly capable of taking you from 5000 right through to a glassy shine. I use a medium cutting compound with a medium polishing sponge attachment on my drill, then move on to a fine cutting compound with a soft sponge and finally a swirl remover with a soft sponge (all on the drill). I move evenly with the same motion and speed as I do when spraying the guitar and tend to do 2 passes with each compound.
With each pass a small squirt from the water bottle is a great way of both lubricating the surface and keeping things cool. With this method the surface looks great even after the initial (medium compound) cutting, but keep going for a perfect mirror shine.
Top tips to remember
Don't start anything until the paint has had ample time to cure
The first sanding stage is the most critical to level the surface perfectly
Use dish soap in your water
Clean up thoroughly before moving to the next grit
Add a few squirts of water for lubrication on the cutting and polishing stages
And finally for those of you that are as impatient as me, a quick tip to test whether your paint is dry before starting out is to take a bit of 2000 grit paper and without wetting it, just run it over the back surface of the guitar a couple of times and then take a look at the paper. If it has started to 'gum-up' a little or if you felt it grabbing the surface at all then your paint is not ready to be finish sanded, leave it a few more days and test again. If it slides nice and evenly over the surface and the there is just white dust on the paper then you're good to go, enjoy!