Ya, to take them out it seems easiest to drill them out. Once you drill them out, you can't use them again, so you would need a rivet gun and some new rivets.
If you cant do that, you can just sand and paint, but you will have a hard time getting into all the hard to reach areas, and you may have a bigger issue with the paint dripping because of that.
You don't "HAVE" to sand it down. But the paint will adhere better and create a better bond. You also won't have as much dripping as you would if you didn't sand. Remember, just sand lightly. Just enough to rough up the surface a bit.
That other panel may be screwed in or rivited in. Not sure. Pics would help on that one.
You can get non glossy clear coats. Clear coats also help to protect the paint. If you don't prep correctly or put clear on, where you screw the screws for your drives will start to wear and chip eventually. May not be a big deal to you, or it may.
Remember to spray in light coats. You don't have to cover everything in the first coat. Spray light, let it dry, repeat as many times as necessary.
The more time you spend on it the better it will come out.
Here is my most recent project.
http://www.thebestcasescenario.com/forum/showthread.php?t=11231
Just started. The mouse is what I want to show you. You can see the steps as I go along. Since that I have finished the mouse with 2000grit and rubbing compound, but haven't gotten around to getting pics up in a while. What is there alone took a week to do on that mouse. So far I've spent another week or so finishing it. It just takes time, but patience pays off. Im no expert, learning to, just passing what Ive learned off to you.
And my keyboards are fine. I just type alot sometimes, too much information to convey on some of these topics.
