I would not attempt to treat the entire inside of the chassis with metal preservative. These product are not generally intended for use on plastics.
If you want to clean the inside, I suggest you start by blowing out the dust, any dirt that remains can be removed using a small cup of water and Windex or similar cleaner.
Use a small (1/2 - 3/4 inch wide) paint brush, the kind used for painting trim, to clean the interior (you must unplug the unit first of course). Don't worry about getting the inside of the chassis wet. Use the brush to dap cleaning solution on any exposed parts or surface that look dirty.
Keep moving the brush from the solution to the chassis and then back to the solution. Don't use this solution on the switches and controls though, use the contact cleaner spray for cleaning these parts.
After you finish applying the cleaning solution using the brush, you will see how much dirt has been removed from the surface of the components and carried back to the container of cleaning solution by the paint brush.
After you have finished cleaning the chassis, you can use the compressed air to blow any remaining solution from around the components, then you must let the unit air dry for at least 24 hours, or longer before attempting to plug it in.
If after cleaning out the dust and dirt there are any true rust areas, you can treat these with a small amount of metal preservative on an old tooth brush. This works well on screws where rust has formed on the head of the screw.
Don't forget to remove the bottom plate for cleaning. Alot of dust and dirt usually settles there. You can leave the feet attached when removing the bottom plate, the feet connect to the chassis directly and there are cutouts in the bottom plate that the feet pass through.
If you want to clean the faceplate, to remove the knobs use two metal teaspoons. Place a spoon with the round side towards the faceplate on each side of the knob. Slip the edge of the spoon under the knob. Press down on the opposite side of each spoon at the same time and this will lever the spoon off the control shaft without breaking the shaft (this of course would be bad).
Be careful when cleaning the faceplate. Don't use any abrasive cleaner or cleaning pad, it will damage the finish. The safest method is to use a mixture of water and Windex (50/50) and a paper towel. Apply this to the faceplate and let it soak in for a few minutes. Wipe it with a towel wet with more solution. Keep at it until it looks clean. Be careful not to rub any of the lettering. If after this treatment there is still a discolored area or an area with scratches, try a small amount of Soft Scrub cleaner on a wet paper towel on the stained/scratched area. Don't rub too hard or the finish may be damaged.