What type sandpaper to use will be decided on based from what you want the finish to look like.
If you want a mirror reflection, it will take a lot of sanding and time with many different types of paper.
What I have done in the past is like this:
I first sand the wood down with 60 grit then 150 then 220. I prime the enclosure usually with an automobile spray on primer. I usually do about 3 coats sanding with 220 lightly between coats. I then apply the first coat of paint very smoothly and evenly. Depending on what type of paint it is will decide whether or not I sand the final layer of paint. Most of the time, I don't sand the paint and just make sure the layers are light and even. I usually use a polyurethane clear coat as it can look good, it's easy to apply, and because it's pretty tough.
With the polyurethane, I start by thinning it out with a little bit of mineral spirits (about 70/30 poly to spirits). I apply the first coat pretty thin and let gravity settle things. I usually do about 2 to 3 coats like this before sanding to make sure I won't sand into paint. If you thin the polyurethane, the coats go on a little thinner but flatten out easier and you get less run lines. Don't try to apply a coat on any vertical surface as you will get runs and they will be a ***** to sand out. After 3 light coats, I wet-sand starting at 220grit until there are no shiny spots left on the surface and without sanding into the paint. I then wet-sand with ~400 girt then 600. After the 600, I put on a few more light coats of polyurethane. At this point, if things have gone well in application, I only need to use the 400 grit and 600 grit. If I'm satisfied with the finish after one more thinned coat of polyurethane, I lightly wet-sand with 600 grit then 1000 grit and then 1500 grit. That should kill all the shine and leave the surface without any deep scratches. I then put the final coat of polyurethane on and when it dries, I will lightly wet-sand with the 1500 grit. After curing for a few days, I use an auto rubbing compound then a fine swirl remover compound and then a wax.
The results can be very nice if you take your time and be gentle.
Here is a picture of a gloss black finish I did on a recent build. It's not perfect but came out **** nice.