[R-390] Painting Knobs

Mon Aug 12 15:48:34 EDT 2024

With knowledge of automotive restoration and architectural paint systems I view this stream with frustration.  I realize this is an area in which fellow hams may not have professional experience, so I want to pass along what I know in case it may be of assistance.  Simply put, you can’t get from here to there successfully with rattle cans, nor abrasive removal of old coatings.  First in the process is obtaining a “base” in which coatings will successfully remain.  You never want to trust prior coatings, even “original” coatings, especially because they may not be compatible with subsequent new coatings.  Always begin with bare metal, but do not go down that road by abrasive means, even with media tumbling on metal knobs.  Any abrasive means will remove original detailing.  (Some more than others, obviously.)  Use chemical means only, following recommended safety measures.

This then brings us to primers and finish coats.  I don’t believe there is an easily applied paint system [primer + finish coat(s)] “consumer” solution which will survive manual turning/manipulation over time by hands not always free of skin oils and perspiration.  There may be one, but I have no experience in such.  A “serious” epoxy finish coat is best, one providing the degree of gloss to match that of the original finish.  That finish coat should be applied over a primer which is appropriate for both the base metal below and finish epoxy coat above.  Please talk with more than one experienced, well respected, locally owned, smaller, automotive body shops about your goals.  Obtain info on their suggested paint system(s) for your knobs' base metal [zinc, “pot metal” (mostly zinc), or aluminum], including the manufacturers and paint names.  Please avoid “etching” primers which may contain acids.  Contact those paint system manufacturers directly to ensure the recommended system(s) is/are appropriate for the base metal(s) and final use (hand-manipulated knobs).  If not, talk with the body shop(s) again.  Chemically remove the old coatings yourself, and deliver the knobs to the selected shop.  

My 2-cents’ worth…

Good luck and 73s,

Bill
AI5RP / KJ5BNE/AE
- - - - - - - - - - - - - - - - - - - - - - 
William H. Blodgett, AIA
Arlington, Texas

> On 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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