Couple of painting tips if painting gloss black...
If you are using MDF, you will have to seal the box using a sealing primer. Regular primer will just soak in the cut edges like a sponge. I have tried using Zinsser and Kilz and got very good results. There is a caveat.. If you are using a lacquer paint, you must use the solvent based primers, not the odor-free ones. The odor-free primers are water-based and tend to crack when lacquer is applied, found this out the hard way. You should spray primer on the entire box. After spraying, sand the box smooth with 220 grit paper. Repeat the process until there is no exposed MDF. Wait a few days before proceeding.
Another note: Do NOT apply lacquer on humid (>65%), cool (<65 degrees) or rainy days. The lacquer will absorb moisture and turn a milky white, this is called "blushing". If this happens, you will have to start all over. I found this out the hard way, AGAIN. Also, do NOT dry your project in the sun. It will cure too fast and cause issues.
I like to then spray three light coats of lacquer over the primer. Wait a couple of hours and repeat the process. The next day, wet-sand the finish lightly with 320-grit. Do NOT use dry paper. The lacquer will not be fully cured and chunks of the lacquer will adhere to the paper, causing mean gouges in the finish. I found this out the hard way. Reapply three thin coats, wait a couple of hours and reapply again. Again, wet-sand with 320-grit paper. Make sure you get rid of any orange peel. If you sand down to the primer, you will have to repeat the lacquer painting process.
You now have a choice to make. You can either spray on several coats of clear lacquer or leave the finish in the black. The clear lacquer will help protect the finish. But the finish of the black will be different with clear. Clear coated finishes will look like it is covered with saran wrap while a pure black finish will have a deep shine, which looks a lot better. The pure black finish is however, very susceptible to damage like scratches which will be easily visible. if you decide to go with the clear coat, apply it like the black lacquer.
FInally, you will need to spray the top coats. Apply three light coats of the clear or black lacquer and let it dry. Now comes the hard part. You will have to wait a month before you can finally finish it. Lacquer takes a while to cure. In cooler weather, it can take a while longer. If you use clear coat, it is imperative that you let it cure before polishing. if you polish the clear coat before it is cured, the heat from polishing will cloud the finish. Again, a lesson from me (I must have thirty coats of paint on my cabinet).
After the paint has cured, you can wet sand the finish with 600-grit, then 800-grit, on to 1000,1500 and if possible, 2000-grit. It's going to take a couple of hours for each level for a box. BTW, you will probably have to go to auto parts stores to find the finer grit paper. At this point, the paint will be super smooth and somewhat glossy. You will now have to use automotive scratch remove and then car polish to get rid of any remaining fine scratches. Then apply a glaze to bring out the gloss. After glazing, use car wax to seal the finish.
I found a nifty product to speed up the process. Woodcraft sells a Micro-Mesh sanding disc pack. It has several pads: 1000-12000 grit which can be used on your 5" random orbital sander. It really works great. You must use a backing pad with this product.
Another BIG tip for you all... When cutting MDF, always make the largest two panels about 1/4" larger than specified. Unless you have a table saw, it will not be unusual for the panels to be a bit "short" when assembled due to cutting inaccuracies. It is easier to trim excess than to make up for missing wood...
Edited by Jon S - 8/9/13 at 1:20pm