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Air dry time between primer coats on a dissimilar metal joint

Aviation Mechanic General (AMG) practice question 33 of 36 on Cleaning and Corrosion Control. ACS element AM.I.G.K23 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONPrimer is being applied to protect a dissimilar metal joint that does not involve magnesium. How long is each coat allowed to air dry before the next coat is applied?

  1. A30 minutes between coats
  2. B6 hours between coats
  3. C24 hours before the next coat is applied
Show the answer and explanation

Correct answer: B. 6 hours between coats

  • A — incorrect No 30-minute dry time is given for this work; the stated interval between primer coats is 6 hours.
  • B — correct The primer for these joints is applied by brush or spray and allowed to air dry 6 hours between coats.
  • C — incorrect A full day is longer than the procedure calls for, which specifies 6 hours of air drying between coats.

BACKGROUND

Contact between different bare metals creates an electrolytic action when moisture is present, and the result is oxidation of one or both metals. The handbook's chart of metal combinations identifies the pairs that require a protective separator, which may be metal primer, aluminum tape, washers, grease, or sealant, depending on the metals involved. All dissimilar joints not involving magnesium are protected by a minimum of two coats of zinc chromate or, preferably, epoxy primer, applied in addition to normal primer requirements. That primer is applied by brush or spray and allowed to air dry 6 hours between coats. Joints that do involve magnesium get at least two coats of primer on each surface plus a layer of pressure-sensitive vinyl tape 0.003 inch thick, or extra primer coats, a minimum of three, where tape cannot be used.

STUDY TIP

Memorize the primer numbers as a set: two coats minimum with 6 hours of air drying between them for joints without magnesium, and 0.003 inch tape or a minimum of three coats where magnesium is involved.

KEY POINTS

  • Dissimilar bare metals in contact corrode electrolytically when moisture is present, so a protective separator is required.
  • Joints without magnesium get at least two coats of zinc chromate or, preferably, epoxy primer, brushed or sprayed on.
  • Each primer coat air dries 6 hours before the next one goes on.

SOURCES

  • FAA-H-8083-30B ch08 "Protection of Dissimilar Metal Contacts", "Contacts Not Involving Magnesium"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-30B Aviation Maintenance Technician Handbook – General, 14 CFR part 43 and part 65. Regulations and handbooks are revised; confirm details against the current edition.