How can I remove the pits from the bottom of my cast iron skillet?

Question

After multiple rounds of sanding with 120, then 80 (because 120 wasn't doing much), 60, and finally with 40-grit, with multiple sanding disks from the two leading brands in my area, my 12" skillet remains pitted.

How can I remove the black spots that you see at the top of the image, on the bottom of the skillet? I'll be content with a rough side wall, but I'd like the bottom of the pan to be as silky as I can get it. The pits (from the sand cast, presumably) are far deeper than I expected, or perhaps the brand (the best-selling in N.A. and one that's normally well respected) I chose is an inferior one after all. Should I go to a professional to get this done right? What kind of shop would I be seeking?

Answer 1

The metal doesn't need to completely smooth to end up with a completely smooth finished pan.

When you season a pan, you're actually polymerizing oil, and basically turning it into a plastic. As you build these layers up, you will fill in any low points. If you use metal utensils when cooking, you will help to scrap off any high points, and you will slowly work towards a smoother pan.

Some people have argued over the years that too smooth of metal on the pan actually makes a less durable seasoning ... there's less surface area, and no irregularities for the seasoning to lock into. I don't know if this is just antecdotal or if there have been any specific studies on this. (smooth surfaces still work, or carbon steel pans would be a problem)

Answer 2

I'm visiting from Home Improvement SE where you originated this question.

The commenters here are telling you that the pits don't matter. As a cast iron pan user myself, I agree with them, but your personal aesthetic demands a smooth bottom in the pan so that the spatula doesn't make an awful scraping noise.

OK, I get it.

Your sanding has been completely ineffective. I don't know if you've used sanding discs on a power tool or if you've been rubbing by hand, but if you're doing it by hand you have about another hundred hours to go.

The tool to use is a 4-1/2" angle grinder with sanding discs on a rubber sanding wheel.

Image: Creative Commons

This tool spins at thousands of RPM and will dig right in to the cast iron and create a deep divot if you're not careful. If you've never used one, you are advised to practice on some scrap steel and get a feel for the tool before unleashing it on the pan.

The process will shoot sparks, magnetic iron dust and abrasive particles in all directions, so it's best to do this outdoors after clamping the pan to a rigid surface. Even outdoors, it's best to wear close fitting goggles plus a full face shield, a particulate mask and hearing protection.

Start with 36 or 40 grit abrasive and work your way up the finer grit sizes once the entire surface takes on an even appearance. I should warn you that you will be removing a lot of iron to get to the bottom of those pits, but it's definitely doable. Most of your expendables will be for your coarsest grit.

I also have to warn you that as you remove material to make the pits disappear, there's a possibility that new pits will appear. This happens when the cast iron suffers from so-called porosity. Cast iron these days is almost never made from virgin ore; iron casting mills melt scrap metal like chopped up cars and trucks, salvaged ships and rail cars and recycled construction waste instead of using new pig iron from a blast furnace. Impurities abound, and these can create voids throughout the casting. Voids are particularly troublesome at the beginning of a production run when slag and dross pour out of the crucible along with the iron.

You can read a well-illustrated academic paper about voids in cast iron here.

You have hours of work ahead of you using the most effective power tool that a home workshop is likely to have. Would your time be better spent shopping for a new pan that's already smooth?

Answer 3

This is not a separate answer. It's a clarification of Joe's answer as I think it's subtle.

On the gritty surfaces that we get on modern cast iron skillets — as illustrated in the top of the figure below — it might take decades to season the pan until all the valleys are covered. That's the volume of the red areas in this cross-section. Until then, the skillet's bottom will remain rough, and you'll feel this roughness when swirling food or when scraping with a spatula. (Please excuse the vulgarity of this quick 3-min sketch; the thickness of the pan is of course far larger than the size of the crests.)

But you don't need to sand the surface until all the crests have disappeared and all the valleys have vanished. If you leave some valleys and sand just enough to bring the dotted line to a lower level, you will then be able to season while adding to the existing seasoned pits. The surface will appear smooth and will feel smooth.

In particular, if you attempt to sand all the way down until you have a truly smooth surface, the material that you need to remove will be so large that an ordinary random orbital sander (with grit-40 or 60) will take an exceedingly long time and effort. You could switch to a faster tool, such as an angle grinder (I personally am uncomfortable with the additional risks of using that tool), but doing so is unnecessary for a perfectly smooth skillet after modest rounds of seasoning.