For long term storage (over winter… it gets c-o-l-d up here!) I spray with G96 Gun Treatment, let it sit about 15 or 20 minutes, wipe off whatever didn't dry and then put a couple coats of Johnson's Paste Wax over that.
If those cast iron surfaces have wood sliding across them, I doubt shellac is going to last very long at all. But for waxed versus de waxed, waxy shellac is only a problem when it's going to be top coated with something that has urethane resins in it (urethanes have adhesion problems), and then only a "maybe". So if you want to do this, I'd say use whatever you have…
Shellac is excellent for sealing a ferrous surface for rust protection, but I wouldn't recommend it for sliding surfaces, such as a table saw top, except for long term storage. It would be great for any other surfaces. Waxed or dewaxed will work in this application. For an alternative, you could use Alox. I have a review for it on this site. For machine tops, use furniture wax w/o silicone.
I bought into the shellac for cast also. Big mistake in my opinion. It took about a gallon of denatured alcohol to remove it. Nothing would slide after application, would leave black streaks on what ever you would slide over it also. I tried it with waxed and dewaxed shellac and waxed the surface after. Now I have gone back to just put wax on and have no problems.
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