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Thread: Primer Woes

  1. #1

    Primer Woes

    Hello,

    My cracked spinner just came back from the repair station looking
    beautiful, and with a yellow tag attached. Time to paint it...

    The process used to be:

    * Clean the aluminum with "Alumiprep"
    * Treat it with "Alodine"
    * Paint it with a zinc-chromate epoxy primer.

    OK, Alumiprep & Alodine are still available. But I hit a snag with the
    primer. What I used to use was PPG DP40, which is a two-part primer
    that
    looks like baby puke and dries hard as a rock. DP40 contained zinc. Zinc
    is important because it likes to corrode. Moisture attacking a zinc-treated
    aluminum panel will prefer to corrode the zinc, and will leave the
    aluminum alone.

    DP-40 is alas gone. The new replacement DPLF-40 is still the proper
    bilious green, but contains no zinc. The can proclaims "No Chrome!".
    The
    bilious green is apparently attained with useless pigments.

    A cursory web search did not reveal any source of zinc-chromate epoxy
    primer. It seems to have been regulated out of the market.
    Zinc-chromate
    lacquer primer is still available.

    If this was an interior part, I wouldn't worry about it much - just prime
    with zinc chromate. But the spinner is in front of the airplane turning
    2500RPM and being blasted by 125MPH winds. Mechanical strength is
    important.

    Looks like I might have to just punt and use the DPLF40 ( actually,
    DPLF48,which is white ) and depend on the alodining and the superior
    sealing properties of the epoxy primer to keep the aluminum from
    corroding.

    Comments?

    - Jerry Kaidor ( jerry@tr2.com )

  2. #2

    Primer Woes

    Jerry,

    Seriously, I think you are obsessing about nothing. Chances are the
    spinner will develop a new crack before corrosion sets in. In 22
    years of flying and owning, I've seen some aluminum (and magnesium)
    corrosion on planes. I've yet to run into a corroded spinner.

    FWIW, I lost 2 spinners and 3 spinner bulkhead to cracks in the first
    2 years I owned my Super (fixed pitch prop). I had my prop overhauled
    a year ago. No more cracks. There may be a connection.

    Best regards,

    Steve Robertson
    N4732J 1967 Super III

  3. #3

    Primer Woes

    Forget the zinc chromate lacquer primer, and don't worry about the lack of zinc. That was far more important in the days preceding the 2-part epoxies. I frequently encountered adhesion and corrosion issues with the zinc chromate lacquer, but have never had a failure (so far) with the epoxy (neither substrate or topcoat adhesion failure, or substrate corrosion.

    Out of curiosity, does the repair preclude polishing your prop? I buffer-polished my Sierra's spinner fifteen years ago, and have maintained it with periodic hand polishing ever since (every few months or so). The polishing is actually less of a hassle than constant paint touch-ups.

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