[R-390] Painting Knobs

[mdmerz]

Jacques, thanks for clarifying alloy of knob. It’s likely that caustic etch is wrong approach since zinc die cast parts are a different animal with their own unique near-surface structure that maybe shouldn’t be disturbed by etching.  Etching may cause other problems but I’m tempted to try it on a spare knob I have here. It hefts heavier than aluminum so it’s probably a zinc-based alloy. But I’m not into repainting so will likely just observe others comments at this point.
Best regards, Dan

Sent from my iPad Air4

> On Aug 12, 2024, at 8:08 AM, Jacques Fortin <jacques.f at videotron.ca> wrote:
> 
> Barry, the knobs of the R-390s are not made of aluminum, it is a zinc alloy
> that is used in a die-cast process.
> I obtained the best re-finishing results (after many disappointing trials)
> by using a Krylon "Dual Superbond" paint-primer product, providing that the
> knobs were thoroughly cleaned in lacquer thinner first and not touched by
> bare hands before painting.
> I do not know if this product is available on your side, but I suppose that
> it is.
> Good luck with your restorations.
> 
> 73, Jacques, VE2JFE in Montreal
> 
> -----Message d'origine-----
> De : r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> De la
> part de Barry
> Envoyé : 12 août 2024 10:07
> À : R-390 Mailing List <r-390 at mailman.qth.net>
> Objet : [R-390] Painting Knobs
> 
> As I've mentioned over the past several months, I'm in the process of
> refinishing the knobs (as well as all the other front-panel parts) for my
> R390.  I stripped, sanded them and used Rust-Oleum Self-Etching primer and
> painted them with Rust-Oleum enamel.
> 
> The Rust-Oleum Self-Etching primer and Rust-Oleum enamel worked very well on
> the front panel, escutcheon, meter covers and, as far as I know, the large
> knobs.  I had reason to remove the paint (again) on the larger knobs and the
> paint and primer stuck quite well; however, I've discovered that the primer
> doesn't seem to stick very well to the smaller knobs.
> 
> While working on the smaller knobs, I noticed a small chip in the paint on
> one of them that exposed the bare metal.  I don't know exactly how that chip
> was made but I think I dropped one of the knobs onto the table top which
> might have hit it just right.  I noticed I could take a sharp putty knife
> and some of the surrounding paint could be removed all too easily.  Another
> knob had a very small spot where the bare metal was exposed as well.
> 
> I decided to dump those two (and a third one that had some small defects in
> the top coat) into a can with acetone and, after a short soak, I pulled them
> out.  The paint had wrinkled quite well and was very easy to remove (it
> virtually fell off); however, the primer also came off very easily which was
> different than some of the other pieces I'd painted.  In fact, the primer
> and paint stuck very well to each other but the entire paint/primer fell
> away from the metal.
> 
> I think those knobs are die-cast aluminum so I can't explain how/why the
> primer wouldn't have stuck to those the same way as the other aluminum parts
> but I just don't know.
> 
> Anyone have a similar experience and, perhaps, have a suggestion for a
> better primer?  I think the factory used conversion (Alodine?) process on
> most all of the aluminum parts and that would probably be a "best" solution.
> I had some parts done that way many years ago around here but it was pretty
> expensive and I'm not sure it's even still an option around here.
> 
> Thanks,
> Barry - N4BUQ
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