Rust Removal from Precision Cast Iron Surfaces in 2020: techniques and when to stop

Hello all.

I'm on a (maybe brief, maybe permanent) machinery restoration kick, and I like to overthink everything before I do it. I'll own it.

Bought a (probably 1950's) E-16 Powermatic planer in running condition that I want to take apart and restore/reassemble for fun. Light to medium rust removal will be part of process for sure, and I've been trying to sift out the facts from the variety of advice out there, often contradictory. Say we're talking about removing surface rust from cast iron tables or mating surfaces like ways...so far I've read things like:

** "if you remove rust from a precision surface, you no longer have a precision surface, changes the dimension, changes the surface"...yes this was from a machinist site probably dealing with ten thousandths.

or

** "surface rust is NOTHING. My contractor saw gets left out in the rain all the damn time. I just take an abrasive pad and rotary sander to it and it's as good as new. I can still see the original grind marks in the cast iron. No problem"

and

** "Dimensionally, the volume/thickness of a rust layer is about 100 times thicker than the Iron layer needed to create it. So removing 100 thou of rust would only remove 1 thou of your original iron" <--that would make some sense to me...don't know if it's true or not though or if the 100 to 1 thing is true

and

** "Tolerance goals for precision cast iron woodworking surfaces tend to be in the single digit thousandths...like +/- 3 thou over a large jointer table. So surface rust can usually be removed without affecting the original tolerances that much or sometimes at all" <--would like that to be true


So can you all weigh-in on your thoughts on any of the above statements...or more preferably...your experiences? Would love to hear 'em.

And what're your favorite abrasive techniques and products (steel wool or scotch brite and what grit level, rotary or rotary/oscll. sander, etc.) and chemical products (vinegar/acetic acid, brand name products like evaporust, muriatic acid, citric acid, diluted hydrochloric acid...etc) that you've tried and like?



And now it's time to go play in the shop. Look forward to your replies and hope you all have a good one this weekend.

-Devin

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There's rust and then there's rust. I've never had any issues as far as what you are worried about, when removing light surface with the usual means (R/O sander with a red or grey scotchbrite). If the surface is pitted, that could be a different story. What does the planer table actually look like?

Erik

My favorite is phosphoric acid (naval jelly as the product in a liquid gel form) with a brillo pad scrub. Following that cleaning with a brillo and some WD-40 has made pretty quick work of fairly rusted surfaces.
I have no tools capable of detecting 2-3 thousandths runout over the length of my jointer table or table saw (or even my band saw or drill press table for that matter).
I've cleaned up a couple of cast iron tables that were in rusty condition but not extreme long term rust. I do have height gages that claim accuracy to half a thousandth (I haven't verified my best one on a calibrated block in quite a while though). I could see no localized variation of even a thousandth of an inch after cleaning them. I don't know about the full table runout. All of my cast iron tables are at least as good and likely better than any of my straight edges that are long enough to span it.
My take on it is rust removal of "typical" rust doesn't impact woodworking surfaces enough to matter. Extreme rust with large pitted areas could but I think it would have to be bad enough that it would already be obvious to you.

Here’s another way to think about it. The rusted surface certainly isn’t as precision as it was pre-rust, and it may not even be functional if moving parts are involved. So you sort of have no choice but to remove the rust.

For the tolerances in woodworking, any of the typical methods are going to give you a surface that is as precise and as function at the original. Naval jelly, scotch brite, evapo rust, electrolysis etc may technically remove surface material but not in measurable amounts that would affect a wood working tool. If you are talking about the ways on a engine lathe where a couple of ten-thousandths would impact accuracy of the tool that is a different story.

I’m talking about surface rust, now if you have actual pitting enough then you still to remove the rust then be thinking about how to repair the surface with fillers, re-machining etc.

My 2 cents.

Torr

Electrolytic or EDTA are the easiest and reasonably fast. Do not use stainless with electrolytic just plain iron or steel. Stainless can make some bad stuff with chrome compounds in the waste.
Bil lD

If you find the finished product (dimensioned boards) out if acceptable tolerances, a sled could be constructed to satisfy your requirements.

My SWAG is that there will be more error induced by cutter runout, uneven roller pressure or blade alignment.

Polish it until it shines!

I take off any heavier rust with a single edge razor blade scraper. Then buffing with maroon Scotchbrite. I never use sandpaper since I don't want to remove metal.

Photos would help. Like everyone else, I’d remove the rust and then see what you have. The table might be dished in the middle from 50,000 sandy board feet. In that case, the rust doesn’t matter and you need to ace the table ground/planed.

I usually take off the heavy stuff with a brass brush mounted in a drill. A grinder would be even better. Once the majority of the surface rust is gone then I switch to an RO with a scotchbrite pad. I think you can do some damage with an RO and 80-100 grit paper, but I don’t think 220-320 would remove a meaningful amount of metal. I’ve never taken a top from rusty to shiny new cast iron. My ‘restored’ tops always have the dull patina of stained cast iron. I’d like to polish the T17’s top to a mirror, so I’m open to suggestions from folks. I might try the scotchbrite pad on the sander with some bar keepers friend or similar.

Razor blade, scotch brite on a cup wheel of an angle grinder. Don’t overthink it.

Barkeepers friend and scotchbrite

Cleaned up the column on an old drill press with a Scotchbrite pad and WD-40 with no problem. Same with some light rust on a table top of a Unisaw I restored.
There are other commercial rust removal products out there for the heavy stuff. Naval Jelly is good but leaves a funny looking protective residue which I don't like.
The least invasive the better; hand power only for the light stuff. Wire wheels in a grinder for the heavy stuff.

Soda blasting does not harm the base material at all. It removes rust and other crud and leaves a light dust on the surface.

Quote:

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Originally Posted by Timothy Orr

...if you have actual pitting...be thinking about...re-machining...

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My reconditioned 8” jointer has pitting of the table surface, as well as dings in the tapered metal pieces at the opening. They are a lighter shade of gray than the tables, but don’t seem as soft as aluminum. They’re no longer available, but a shop could surely fabricate those, couldn’t they?

How do you find somebody who can do that work, and do it well? When I’ve called machine shops, (a hobbyist, not a pro) my amateur way of describing what I need either confuses the guy, or convinces him that he’d be wasting his time.

How do you choose a shop, then describe resurfacing a jointer surface? What’s a fair price?

ROS with grit to match the work required. Better to get thru it in a few minutes than to go very slowly and spend a long time. The faster approach will encourage you to finish all the surface the same rather than focus on the bad spot.

You would have to do some serious rust/base metal removal to make a difference in flatness measurable by any normal tools in your shop. I doubt you would remove even a few thousandths unless you wanted to remove all traces of serious pitting..