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Classic Repair #1: White rust on metal chassis

S

stephenbuck

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Jun 21, 2019
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Location
Boulder, Colorado
This is the first of what I hope will become a series of posts on restoration techniques for vintage computers if there is enough interest. I'm working on several restoration projects and constantly run across issues that I'm sure the collective wisdom of VCFed can help with. I'll be sure to show the before, during, and after photos.

For this restoration, I've encountered quite a bit of what I'm assuming is "white rust" on a DEC BA11-N chassis for a PDP-11/23. There is also some other form of corrosion where it looks like there has been some wear. Hoping to solicit some suggestions on how to proceed with cleaning this up.

My best guess is to use vinegar and a scotch-brite pad to remove it and then either apply clear-coat to protect it or apply a "galvanizing" paint. Thoughts?

In general, I prefer to do as little as possible in the way of permanent changes, but am not opposed to something like painting if it's absolutely necessary.

Here's a link to the restoration project: DEC PDP-11/23+ Restoration | Head Spin Labs (wordpress.com)


Thanks, Steve
 

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Galvanized steel chassis? The white stuff is zinc oxide--moisture and time have caused the zinc galvanizing to react with the air.

For restoration, you have a couple of options:

1. Just buff it off and clear-coat it. Not as good as galvanizing, but it might do.
2. Replate the part with something like Caswell zinc plating kit.
3. Use zinc-containing paint, such as Rust Oleum cold galvanizing spray. The finish is matte, not shiny, however.

I've also successfully done some zinc electroplating plating using zinc sulfate from the feed store (used as moss killer) and a zinc metal anode.
 
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Hi Chuck,
Thanks for the advice. I think I'll try buffing it off and then try the cold galvanizing spray you mentioned. That should work for the front bezel brackets. The other pieces, particularly those with lettering, are a bit tougher. I may try some spot removal with vinegar and then use the clear coat. Will post photos!
Thanks,
Steve
 
Vinegar and a gentle scouring took the white rust off the front bezel brackets quite easily, but it started to reappear almost immediately. Will try the cold galvanizing paint you mentioned later today. The surface looks rough, but is relatively smooth.

Thanks,
Steve


View attachment IMG_1671 (1).jfif
 
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Even though it's a high metallic zinc content, this is still paint, so softer than solid metal. So proceed accordingly. Personally, I'd put a scratch brush wheel on my buffing setup and work from there.

The zinc sprays have an interesting side use--when a non-metallic case is used, spraying the interior of said case reduces high-frequency EMI. Carbon and nickel were also used in this application.
 
Here's the bracket with two coats of the "cold galvanizing" paint. Definitely flat, like you said it would be, but a giant improvement. I wonder if some clear coat might gloss it up a bit? Chuck, are you recommending the brush wheel to remove all of the original zinc coating?

Thanks,
Steve


View attachment Zinc.jfif
 
If you use a brass scratchbrush, it shouldn't affect the existing galvanizing, but give the part a bit of a shine. If you go to a steel wire wheel, you'll definitely remove the galvanizing--and then you can clear-coat it and it will still look pretty good.

Really, electroplating is the only way to restore the zinc surface.
 
Thanks for all your advice Chuck. I'll live with the "galvanizing" paint for now but, as you stated, the best solution (other than electroplating) would have been to remove the galvanizing and clear coat the surface. I've never had something electroplated - is it difficult or expensive. The DIY route makes me think of hazmat suits!
Steve
 
In the universe of electrofinishing, zinc is among the easiest metal to plate--on steel there's no "strike coat" required. There are lots of DIY videos on Youtube, most for small parts. As in all plating, you have to be obsessive about cleanliness. Degreasing and activation (pickling) are important. Beyond that, you need a large enough container and a variable output DC supply and a few chemicals. Forget the DIY ones that use vinegar--you really can't get a bright, dense finish.

Because I live in an area where moss is a perpetual wintertime plague, I pick up zinc sulfate from the local farm supply in 50 lb. bags. most gets sprinkled over moss, but there's always some remnant. For the anode, I have a stock of scrap zinc, but if you don't have that, know that zinc "sacrificial' anodes are sold for retarding corrosion on water heaters and buried pipes. Sulfuric acid is pretty easy to come by--it's battery acid. Like all chemicals, wear protection and work in a well-ventilated area is essential. I've only used acid plating, but there is an alkaline plating process as well (there's a YT video of a bloke demonstrating his 10 litre kit).

The pros use a cyanide solution, but in the case of zinc, it's not necessary for good results.

More than 50 years ago, I worked in a steel mill as an instrumentation technician. Electrogalvanize lines there were perhaps half a city block long with vats of nasty chemicals. Essentially a 20-30 ton coil of steel sheet fed in one end and nice shiny zinc plated steel coiled up on the far end--a continuous process. Tin-plating lines were similarly configured.
 
I have had little to no luck with the cold galvanizing spray. It never matches the finish and it comes off pretty easily with the slightest scrape ir hard rub. You would need to respray the entire unit if you want a uniform look or as Chuck pointed out do an electroplate. Best and easiest may just be to paint once corrosion is removed. That is my goto.
 
I've had great luck with the rusto galvanizing paint. I've only used it in outdoor applications where finish sheen is irrelevant but it seems very rugged and durable when properly applied
 
Properly applied? Whats the insinuation? Did I try and apply it to greased pig? Cleaned surface,,, shake well, spray evenly. I think I got that part checked.

Forgot to mention the price was more than other Rustoleum products.... keep that in mind.
 
yeah, I thought so. I assure you the surface was adequately prepped. I am only relaying my experience with the product.
 
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