I recently received the following question from a reader:
I have a cast-iron table saw that hasn't been used for a few years, and has accumulated a pretty substantial amount of rust on the table. How can I remove the rust and keep it from coming back?
Well, there are about as many answers to that question as there are woodworkers who have faced the problem. Cast iron tables are great for their weight and durability, but they're quite susceptible to rust, particularly in high-humidity climates.
The first step is assess the situation. If the rust is relatively light and superficial, it won't take much to clean up the problem. However, if the rust has been accumulating for a number of years and appears to be substantial and deep, the saw's table is probably too damaged to return it to its former glory, so to speak. That being said, the saw should still be usable - it just may not be as smooth and precise of a surface as when it was new.
First of all, resist the urge to go after your saw table with sandpaper. Instead, try a Scotchbrite or similar synthetic kitchen scrubbing pad (not steel wool) with some mineral oil as lubricant. You want to use a sufficient amount of mineral oil, but don't go to excess, as you'll have to wipe off the oil when the scrubbing is done.
I've heard of people using WD-40 instead of mineral oil, which should work fine, although it will evaporate more quickly than mineral oil. Should you choose to use the WD-40, apply it often and to the entire saw table to keep it lubricated until you've finished scrubbing.
For heavier rust, you may be able to use your random orbital sander to help with the scrubbing. Place the base of the sander on top of the scrubbing pad and get to work.
A completely different idea that has some merit for extremely rusted cast iron tables is to use naval jelly. You'll need to follow the instructions on the naval jelly to the letter, particularly when it comes to removal, so as to avoid pitting the cast iron. Naval jelly is quite aggressive for rust removal, but it needs to be removed properly to stop the cleaning action.
Once the rust is removed, you'll want to take steps to ensure that the rust won't return. There are a number of commercial products that not only protect the table from rust, but also lubricate the table to make the stock slide better when cutting. T-9 and Slipit are popular choices, typically available online or at your fine woodworking supplier.
Paste waxes also work well for protecting and lubricating, but avoid the temptation to use car paste waxes. These formulations typically contain silicone, which will plug up wood pores and cause issues with finishing your stock. Instead, try some Johnson's Paste Wax (which is typically formulated for floors). Apply liberally, wait until it is nearly dry and then wipe the table down to remove any excess.
One final note. It may be an old wives' tale, but one that I tend to believe: plain old distilled vinegar can act as a rust inhibitor. After the majority of the rust is removed using your method of choice above, wipe down the entire table with a liberal amount of vinegar. Allow it to dry and then apply your final protectant/lubricant. The acidity of the vinegar should help clean up any last little bit of rust that you may have missed, and should also impede the future development of more rust.
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