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

[Devin Brenan]

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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[Erik Loza]

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

[Bruce Mack]

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

This is a great thread. My Ridgid jointer has rust pox. Where can I buy scotch brite 5" hook and loop discs? I see them on line in quantity, but I'd like to buy minimally abrasive discs in single or 5-pack quantity, locally preferable but online is OK. Thanks.

[Eric Arnsdorff]

This is a great thread. My Ridgid jointer has rust pox. Where can I buy scotch brite 5" hook and loop discs? I see them on line in quantity, but I'd like to buy minimally abrasive discs in single or 5-pack quantity, locally preferable but online is OK. Thanks.

Bruce you don't need the hook and loop disc. The grocery store variety scotch brite pads will work fine with the RO sander.

[Bruce Mack]

Great. I have those in shop. Thanks!

[Eric Arnsdorff]

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.

[Timothy Orr]

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

[Bill Dufour]

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

[Devin Brenan]

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

Thanks for the reply...I will google it. Don't think anyone else brought this method up. How or on what have you used it before?

[David Kenagy]

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

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?

[Devin Brenan]

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?

I read an article or two about this over on the vintage machinery site. They have a wiki section. Look for sections 2.8 Jointers & Planers (Grinding Jointer Tables) and 2.11 Restoration Tips ( Flattening Cast Iron Surfaces)

[Devin Brenan]

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

Thanks for the reply...and yea point taken about the rust..it'll have to come off no matter what I suppose and then I can assess where the table is at. As of right now when I measure it with the tools I have, it's showing a ~ 6 thou wallow in the center of the table that'll probably be there-to-stay unless I have it resurfaced. I may decide to live with the wallow though, tbd.

[Devin Brenan]

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.

Eric, thanks for the reply. Only recently heard about naval jelly. Glad to hear that works for you. And...

"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." <--that's also a good point.

I do have a 48" Starrett straight edge (model 380-48) that claims an accuracy of +/-0.0002 per foot, or +/- 0.0008 over the whole thing with 1.6 thou worst case...but I have no way to measure that with a granite plate or anything. And the square I used to measure the table in a later response is .001 per inch advertised. Anyway, the more responses I get similar to yours, the less and less I'm caring about single-digit thousandths...I just thought why not aim for it if I can since they recommend setting your knives within 1 thou across the table anyway.

[Jim Matthews]

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!

[Richard Coers]

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.