Ideas on restoring a corroded dial?

[TerraHertz]

So I bought the three HP instruments TPI advertised. See https://www.eevblog.com/forum/buysellwanted/(fs)-hp331a-hp334a-hp654a-signal-gen-distortion-meters/
Thanks for those TPI!

Now there's the problem of what to do about the corroded main dials on the HP 654A, and 331A.
The 331A's dial is engraved, so that should work out OK.

But the 654A ... I'm not sure how to go about it. The dial is quite large - 105mm dia. The surface was originally a matte anodized aluminium, with black lettering screened on, then coated with a clear lacquer. For whatever reason, the lacquer has failed in spots and squiggly patterns, and the exposed aluminum corroded. It seems to be only surface corrosion - no pitting.

Making up a new artwork in Illustrator shouldn't be hard, but the problem with getting it printed onto the dial is that the dial is not flat. It's a cone, with the center raised about 5mm.

I know I can get the metal back to a nice even matte finish, and make a new artwork. It's printing the legend on that so far I have no clue how to do.

Anyone have any ideas?

[Andy Watson]

Waterslide transfer?

[robrenz]

If you prepare the artwork in the right format having a laser marking service mark the dial would work well and not cost too much. I would suggest  a light glass bead finish on the aluminum after you get a uniform sanded finish.  Many automotive machine shops will have a glass bead cabinet for cleaning heads etc.  You could get your parts done for a cup of coffee. You want them to do it at a relatively low pressure (30-40 PSI) You would want to take an aluminum scrap piece that was glass beaded also for the laser shop to get their settings right for the type of mark you are after.

If you don't like this idea I have more.

[Psi]

Here's what i would try. (it requires a steady hand which most engineers have due to skills soldering smt)
It takes advantage of a problem ever painter comes across, that paint is a bitch to get out of cracks/grooves.

1) Get a dremel or ideally an engraver with an ultra fine engraving tip and use it to go over every bit of black lettering and all the lines.
(So they're now a groove in the metal).

2) Sand the whole thing with ultra fine sandpaper to remove the rust/corrosion but keep all the grooves left by the engraver.
(The sanding has to be ultra fine so you don't introduce deep grooves in the main area, i would try 1000 grit)

3) Spray the whole thing with a little black spray paint and then immediately wipe it with a cloth. The paint will collect in the grooves but should wipe off the main areas relatively easily.  (you might need to repeat this a few times and you can use a little turps on the cloth if necessary).
Alternatively you could try wiping paint onto the grooves instead, and clean off excess with another cloth.

4) Once you have it looking black where black should be, and clean where clean should be, let it dry and you should have a nice looking dial again. (you can coat in clear if desired)

Note: I've tried something similar to this in the past with good results but it's always a bit of a gamble.
The lettering wont come out looking as sharp as printed text, since the grooves were cut by hand, but it should look good enough if you have a steady hand and follow the current lettering exactly.
The lines should be pretty easy to do

[TerraHertz]

If you prepare the artwork in the right format having a laser marking service mark the dial would work well and not cost too much. I would suggest  a light glass bead finish on the aluminum after you get a uniform sanded finish.  Many automotive machine shops will have a glass bead cabinet for cleaning heads etc.  You could get your parts done for a cup of coffee. You want them to do it at a relatively low pressure (30-40 PSI) You would want to take an aluminum scrap piece that was glass beaded also for the laser shop to get their settings right for the type of mark you are after.

If you don't like this idea I have more.

The frustrating thing is that I have a sandblasting cabinet NIB, big compressor, and all the bags of various grit, balls, etc, but all are buried under a junkpile in a still-incomplete shed that they'll eventually be used in. Oh well.

I'll see what I can find in the way of laser marking services. Ideally I'm hoping to come up with a method that's free. But still results in a photo-perfect finish.
There's also a small attenuator dial with rubbed-off numbers, that I'll probably just Letraset by hand then lacquer.

Psi: you must have superhuman steady hands. I can solder fine components OK, but trying to engrave a visually flawless version of this by hand... nope. Not going to happen. It's also a 'one chance only' thing, since pressing a new dial shape from sheet aluminium, while possible, would be a pita.

Edit: Hmm... though, I _could_ mount the dial on my lathe or rotary table, and rig the engraver to do the radial gradations. But there are three different line lengths, so making a slide & endstops to get them all exactly right... urgh.

The numbers could be Letrasetted on.  Ah... though then there's a slight sequencing problem with the letraset and black paint in the gradations. I guess paint first.

[Psi]

Psi: you must have superhuman steady hands. I can solder fine components OK, but trying to engrave a visually flawless version of this by hand... nope. Not going to happen. It's also a 'one chance only' thing, since pressing a new dial shape from sheet aluminium, while possible, would be a pita.

It works better than you would think.
But yeah, definitely a one shot.

Being 105mm wide the lettering isn't too small to trace by hand.

[TerraHertz]

Being 105mm wide the lettering isn't too small to trace by hand.

If I wanted this to end up looking as shitty as my hand lettering, I'd just leave it the way it is now.

[robrenz]

If you are ok with the letraset then how about this,

Make a plug that slip fits the center bore of the dial and is 1mm longer than the height of the dial.  wood, plastic, metal, whatever. Super glue a strip of metal about 75mm long (straight edge) on the plug with one edge intersecting the centerline of the plug. Take a 150mm disk of nice cardboard with a good writing surface. Use a couple small strips of double stick tape on the backside of the dial to keep it from rotating when centered on the cardboard. Put the plug/straightedge in the bore and now take a ruling pen and transfer all the existing dial graduation lines to the cardboard. Make the lines similar long short lengths on the cardboard as a guide for when you use this as your template. Also mark the corresponding numbers on the cardboard lines for later reference. Now do your dial surface prep and put the dial back on the cardboard template and use the plug/straightedge with a ruling pen with ink or paint to put the graduations back on the dial using the cardboard lines for reference. When dry use the letraset to do the numbers. Cover with your favorite clear spray to seal. make sure whatever you use does not dissolve the ink/paint/letraset.

[DmitryL]

Making up a new artwork in Illustrator shouldn't be hard, but the problem with getting it printed onto the dial is that the dial is not flat. It's a cone, with the center raised about 5mm.
I know I can get the metal back to a nice even matte finish, and make a new artwork. It's printing the legend on that so far I have no clue how to do.
Anyone have any ideas?

I would try this:
- print your dial on a transparent laser film normally (not mirrored)
- cut it as you need
- take a UV "glass glue", it is extremely transparent
- put a thin layer of this glue on your dial and distribute it evenly
- put your transparent film on top with printed side to the glue
- remove the air bubbles
- cure the glue with UV light or direct sunlight
- enjoy

[edpalmer42]

Imagine if you lightly stuck a piece of paper onto the dial, trimmed the edges neatly, and then peeled the paper off.  You'd have a flat piece of paper cut into a funny shape.  Could you figure out how to print the artwork on that flat piece of paper so that it would be correct when stuck back onto the dial?  You would eventually print the artwork onto a transparency, decal, or whatever.

Ed

[Psi]

Could attempt a toner transfer onto the metal, as if it was a PCB, then clearcoat.

[TerraHertz]

If you are ok with the letraset then how about this,

Make a plug that slip fits the center bore of the dial and is 1mm longer than the height of the dial.  wood, plastic, metal, whatever. Super glue a strip of metal about 75mm long (straight edge) on the plug with one edge intersecting the centerline of the plug. Take a 150mm disk of nice cardboard with a good writing surface. Use a couple small strips of double stick tape on the backside of the dial to keep it from rotating when centered on the cardboard. Put the plug/straightedge in the bore and now take a ruling pen and transfer all the existing dial graduation lines to the cardboard. Make the lines similar long short lengths on the cardboard as a guide for when you use this as your template. Also mark the corresponding numbers on the cardboard lines for later reference. Now do your dial surface prep and put the dial back on the cardboard template and use the plug/straightedge with a ruling pen with ink or paint to put the graduations back on the dial using the cardboard lines for reference. When dry use the letraset to do the numbers. Cover with your favorite clear spray to seal. make sure whatever you use does not dissolve the ink/paint/letraset.

I really like your idea of using a plug-centered radius ruler to transfer the gradations to a back-sheet, then transfer them back to the dial once it's clean.
Too bad I already sanded the markings off!
Never mind, I can print the photo of the dial a bit oversize to get a back-sheet with the lines around the edge of the dial. Carefully took the pics as exactly on-center and perpendicular as I could get (tripod and plumbob), so there shouldn't be much distortion.

Turns out there is some slight pitting of the surface. So I'm going to have to be a bit more aggressive with the sanding than I'd hoped. Hence, mounting it in the lathe:

[robrenz]

Glad I could be of help.  If you have any of the old rapidograph inking pens they would be great but if not the old drafting set ruling pens work great and will work with thinned paint just as well as ink.

[robrenz]

I have some vague memories of clear coating letraset from way back I think the key was to do very light single pass sprays and let each one dry very thoroughly. Don't worry about it wetting out until your final coat.  Even though the clear may dissolve the letraset if left wet long enough, this method has the solvent evaporate quickly before it has time to work on the letraset. After you have a few light coats that are well dried and have sealed the text you can do a final coat that just wets out.

An extremely durable and high quality clearcoat is Permalac extremely expensive but extremely good stuff.

Edit: I bought 2 aerosol spray cans but if I ever need to get more I would buy it in a non aerosol can and spray it with an airbrush. It sprays very heavy and wet out of the aerosol cans and is very tricky to not get runs on intricate things like scope bezels and frames.