[R-390] Painting Knobs

[Barry Scott]

Some of the knobs had a noticeably dull gray finish under the paint and now
I wonder if that was from an etching primer.

Thanks,
Barry - N4BUQ


On Mon, Aug 12, 2024 at 12:25 PM 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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