Zinc coated metal repairs

[dominicbeesley]

I've got a number of items, mainly power supplies from older computers and a few CRT chassis, that have signs of rust on them. These tend to be made of zinc coated steel (Zintec?). The attached example is from an Acorn RiscPC power supply which got leaked on in storage. The monitor that was next-to it was similarly affected.

I'm not sure of the best way to repair them. These don't need to be concourse standard nor do I particularly want to lose all the patina with a full paint job but I'm not sure what to do to effect a repair that doesn't involve a complete strip down and paint with something way too shiny. There doesn't seem to be much on the net other than people wanting to bling their PCs but this must be a relatively common occurrence?

I've thought about zinc-rich car primer but would that be effective long-term. I've had experience with primers going rusty pretty quickly if a top-coat isn't applied but that has been on outside projects.

Suggestions welcome.

D

[trevatxtal]

Toolstation stock "Plastikote", "Action Can" some of the many Zink coatings to cover welded joints on galvanize metal.
There are many other products.
The following will give you a way of doing it.
https://www.galvanizing.org.uk/weldi...vanized-steel/

[G6Tanuki]

Primers are generally porous so oxygen and moisture can still get through to the underlying metal unless you seal the primer with a topcoat or a clearcoat.

Either way, you need to get rid of all the rust and corrosion before any repainting. Pickling in Coca-Cola is one way - it contains Phosphoric Acid which slowly digests the rust. Other people have tried elkectrolytic de-rusting techniques involving baths of Lye or similar caustic material and a 12 volt car battery connected to the rusty item. I've not tried this.

[G6ONEDave]

Start by removing the loose rust after that I would heat up the metalwork to the point where steam is emitted. When the steam stops, remove the heat. Do this outside and don't breathe any of the vapours in. Once the metal cools down again remove any rust that is now loose. Next paint some Fertan or equivalent chemical onto the previously rusty area. This should turn black when dried. Clean off any excess when dried and then prime it with suitable paint. If this is done correctly, the previously rusty area should remain rust free. Be careful not to over heat the zinc coated areas as you don't want to breathe the emitted sulphur fumes.

The Fertan will chemically convert the steel to be almost gun metal. It should also prevent the converted steel from going rusty again. Don't leave any bare steel for long as otherwise it will start to rust again from the action of Oxygen and moisture in the atmosphere around the steel.

Make sure that you follow all health and safety precautions and any instructions on the products used, as this can be a dangerous process if not done carefully.

There are various other methods and chemicals available, this is just my suggestion that might be of help.

Dave

[The Philpott]

I was considering mentioning the 'cold galvanizing' aerosols mentioned in Post No.2, but it's unlikely to get you a colour match. Preferable to primer for sure, as per previous comments.

I see from the photo that on that particular assy. the zinc has to some extent succeeded as sacrificial anode and protected the steel. One hopes the pitting in the steel wouldn't therefore be too deep to remove by mechanical means, you could then use phosphoric as belt & braces afterwards.

I've only ever found Jenolite or other Phosphoric treatments to be effective if ALL rust is removed beforehand.

Dave

[Uncle Bulgaria]

If you want to touch it up to match, then you need to know what the other coating is. If Zintec or equivalent then it's electroplated zinc which you're not going to match unless you plate some more on yourself.

If it were me, I'd get rid of the rust using an angle grinder/die grinder/dremel thing depending on the size of tool and viciousness required, with a non-woven surface conditioning wheel or wire brush. If the latter, scuff up the surface with some reasonably fine sandpaper before painting with phosphoric acid/fertan/kurust/to your taste, neutralising if required by the product, then painting with zinc phosphate primer followed by a top coat.

Yes, primer is porous to some extent, but these are objects that are going to be inside and not exposed to great extremes of temperature that might cause condensation. Assuming your house is not a health hazard and very damp, then you will have removed one of the two elements required for rust: moisture, and they won't suffer again. I've had some car bits outside in Bondarust for a couple of years and they're OK. I hope your computers won't be treated similarly!

A Halfords zinc-rich aerosol will give a similar grey colour, and if you want to top coat or not, depending on whether you think that will make it look too new, it will certainly look better than it did when rusty even just in the primer.

I've got some Paragon Paints zinc-phosphate primer which is nice to apply and leaves a flat grey finish.

[Bazz4CQJ]

As a paint to deal with metal which already has some rust on it, I am a huge fan of zinc phosphate primers. These seem to do a great job, always accepting that once rust has started, there will be pits which won't easily be removed by sanding or similar.

The primers containing zinc metal tend to be mucho expensive and a bit thick and stodgy.

B

[G6Tanuki]

Originally Posted by Bazz4CQJ

As a paint to deal with metal which already has some rust on it, I am a huge fan of zinc phosphate primers. These seem to do a great job, always accepting that once rust has started, there will be pits which won't easily be removed by sanding or similar.B

The aviation industry likes Chromate-type etch-primers for dealing with pitted metal; though you really need to make sure you remove *all* previous oxidisation-products from the surface - which introduces its own range of chemistries.

Alas the current H&S rules [REACH et al] tend to be rather prohibitive of the sale of nicely-aggressive etch-primers to the general public. There are some alternatives freely available, like U-Pol Acid#8 https://www.halfords.com/motoring/pa...er-741124.html - - but it's nowhere as good as the stuff we used in the 60s and 70s.

[Bazz4CQJ]

Originally Posted by G6Tanuki

The aviation industry likes Chromate-type etch-primers for dealing with pitted metal; though you really need to make sure you remove *all* previous oxidisation-products from the surface - which introduces its own range of chemistries.

Yes, but these days, I'm not sure whether obtaining chromate paints is just very difficult, or plain impossible. There are those who are compulsive about the use of epoxy paints, but I note that in recent years, quite a few epoxies now have zinc phosphate added to them.

B

[dominicbeesley]

Thanks for all the replies.

I've made a start with getting rid of the rust. Some areas are quite pitted but not so pitted that they need filling. I've given it a good rub down over the affected patches and soaked treated them with coke for now but will try and find my bottle of fertan. I'm down to bare metal except a little hard black stuff which is quite tough. I may go the whole hog and attack that too but it really seems quite stable and takes a sharp file to shift it.

I think as an experiment I'll go with the least-effort path of giving it a coat of zinc rich primer....I have a can left over from a filing-cabinet respray which if memory serves should do the trick. I'll be hanging on to this machine and it will be a nice experiment to see if I deteriorate quicker than the primer! If the colour/finish isn't a good match then I'll just have to maybe spray the whole lot.

The reason I've been put off primer only is that on car/exterior jobs the paint hasn't lasted well but then again it's unlikely to be sat in a damp field after a dousing of road salt. Also, hopefully the computer won't be left sat in a puddle of sewer water this time...The rest of the computer is in a sorry state having also been host to a Varta battery and the plastic case being left in front of a fan heater by an eejit...but that is for another thread or threads!

D

[G6ONEDave]

When I first used products such as Kurust and Fertan I was never successful in preventing the rust from re-appearing. I was then informed that unless I heat the metal up until it gives off the steam/vapour it will not last. It turns out that steel despite being a solid material is actually like a sponge at micro levels and it's this sponge effect that holds moisture, hence the need to heat it up to dry it out. It does sound odd but that's the way of things. I'm sure that a chemist or metalogist will be able to explain the process involved in greater detail. It never mentioned that the metalwork needed to be heated to dry it out on the product containers.

Dave

Dave

[ex seismic]

I can recommend electrolysis, it will clear out the pits. I use a 5v computer PSU which works quite happily.

[Nickthedentist]

Tell us more...

[ex seismic]

The workpiece is one electrode, a stainless steel plate the other. I forget which way round, google is your friend here. Washing soda as the electrolyte. Apply DC of choice, sit back and watch the bubbles! The SS plate will get grubby.

[mark_in_manc]

I might add something here. Work is cathode, anode is sacrificial. Some folks online say that using a stainess anode gives off some nasty chrome compound which you don't want to be around. I use steel. I once used plasterer's mesh (as eletrolysis is a line of sight thing, and I wanted an anode to curve around a complex work piece - bicycle forks). It worked well in terms of rust removal, and also deposited the zinc off the galv mesh, onto the workpiece - that might be an interesting thing to try here! I use a caustic soda electrolyte, which I suppose is a bit more aggressive. You can clean your drains with it afterwards, or keep it in a safe container for next time.

[Uncle Bulgaria]

Yes, stainless steel anode is the setting for an Erin Brokovich moment - hexavalent chromium. The same stuff in zinc chromate paints, probably why they're hard to get hold of. Not nice at all, notifiable and probably impossible to dispose of without getting a hazardous waste company involved.

For a reduced-sludge affair, use carbon anodes. Rods of the same are available as 'graphite stirring rods' for jewellers, to use with molten metal. It will work out of line-of-sight, but since the path of least resistance is direct, it's a lot slower to do the back of something if one doesn't have anodes dotted around the perimeter of the electrolysis tub.

A sodium carbonate electrolyte is a bit friendlier than caustic soda. As mark_in_manc found, caustic soda works well, but is more aggressive and rather more unpleasant to be around.

[jackrae]

For electrolysis how about using a galvanised steel pail as a combined container and anode in one. Then suspend the item to be de-rusted from an insulated (wooden) rod across the top of the pail

[matthewhouse]

Originally Posted by ex seismic

sit back and watch the bubbles!

The bubbles are really loud if you ignite them!

[ScottBouch]

A slightly less toxic method: Sand blast off the rust and old plating, then apply new plating? If you have several items it may not work out too expensive per item to have this done professonally to order.

Years ago I did a lot of sand blasting of car parts (long rubber gloves in a sealed cabinet), it was an incredible way of removing rust and getting back to fresh metal. I was painting these components - and had to be quick with the paint before the fresh metal starts to rust. Also no touching the blasted metal with bare hands, as finger prints show up instantly!