[R-390] Painting Knobs

Mon Aug 12 18:28:28 EDT 2024

The density of my spare knob  is over 6 g/cm3 so it’s definitely not aluminum and that is a density typical of zinc die casting. The knobs on my R390 look the same but my R390a knobs are too pristine to compare visually so maybe I’ll find the wrench and take them off for weight measurement.  I see no reason it would not be zinc die casting also.  Ok I found the wrench and took knobs off the R390 and much later R390a. They have the same density indicative of zinc die casting alloy.  I did notice the R390a knob didn’t have the cast numbers on the underside and had the coating there whereas the other two knobs were not coated on the underside.
 The term “pot metal” was likely used to describe backyard foundary products. The quality of zinc die casting metal can be very high and depends on what the producer puts in.  Small amounts of some impurities cause cracking over time but this problem was identified and known before R390 knobs were manufactured.
Dan

Sent from my iPad Air4

> On Aug 12, 2024, at 11:18 AM, Jim Whartenby via R-390 <r-390 at mailman.qth.net> wrote:
> Has the composition of the knobs changed over the many contracts of R-390 and R-390A receivers from cast aluminum to pot metal?
> JimLogic: Method used to arrive at the wrong conclusion, with confidence.  Murphy 
> 
>    On Monday, August 12, 2024 at 12:25:58 PM CDT, mdmerz <mdmerz at frontier.com> wrote:   
> 
> 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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