I think you may be disappointed with results if you approach your shielding work with the idea that RF behaves like optical energy, that is, light rays. For example, a sheet of metal, say 1ft x 1 ft, would stop a light beam from a laser pointer pretty effectively, but it will do almost nothing to stop an RF signal at 800 MHz. The reason is that the piece of metal is fairly small, and RF energy will sweep around it more like waves of water than beams of light. The RF will be "stopped" in the center of the metal by inducing a current in the metal, but then this current will conduct out to the edges, flow around to the other side, and in the process reradiate. So it will appear to pass right through the metal. Some RF will also refract off the sharp edges of the metal and bend inwards, also filling in behind the metal. You have to imagine RF energy as more like waves of water rather than beams of light, and remember the old physics lab experiments where you observe waves in a shallow wave tank and how they bend around things.
When visualizing that old wave tank, imagine that the distance between the wave peaks is, say, 1 inch, and your shielded box is also about 1 inch across (analagous to a 12 inch box reacting to 800 MHz waves). The box in this case will be almost transparent to the waves as they sweep around it and fill in behind it.
So, the effectiveness of a metal shield made from a good conductor is dependent on the size of the shield compared to a wavelength. If your box is relatively small in wavelength terms, then the RF will sneak around it looking for any opening, then go through that opening. This is especially true if there is a wire passing through a hole. That wire makes a much lower impedance path for RF to sneak in than a simple hole presents. This means that not only do you have to worry about holes and slots in your box, but you have to be 100 times more worried about wires that go in and out of that box.
So, if all you do is paint a box, but not seal up the seams and holes with metallic conductor gaskets or such things, then the RF will sneak in/out anyway and you will be dissappointed. If you don't use filtering for each conductor that passes in or out of the box, you will also be dissapointed with your paint job.
Doing a proper job of protecting a circuit inside a box from bombardement by UHF rf energy is an advanced art. There are many details I have not mentioned.