Filling Rivet Holes

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Overview

Blind rivets leave an open mandrel hole in the skin. Filling them before paint is a finishing step, not a build step — the Van's KAI does not mention it, and pulled rivets installed per the plans are structurally complete without it. Builders fill for three reasons: a smoother painted surface, keeping water out of the cavity, and covering the broken steel mandrel stem left inside the rivet, which can rust over the life of the airplane depending on climate and storage.

One DAR characterizes filling as normal paint preparation — comparable to filling pinholes in composite — rather than a deviation from the build. (VAF)

Deciding whether to fill

  • For: covers the steel mandrel stem; smoother surface under paint; most builders who have done it say they would do it again. (VAF)
  • Against — paint adhesion: filler that does not bond in a dusty, oxidized cavity can crack the paint over it later. (VAF)
  • Against — future rivet removal: a builder drilling rivets out for a Van's service bulletin had to dig filler out to find the rivet centers, and damaged paint doing it. (VAF) Counter: the filler is soft compared with aluminum, so a careful drill still centers. (VAF)
  • Time: roughly 1 hour per wing side and 2 hours for the tailcone, stabilizer and rudder with a dispenser (VAF); one builder did a VS, flap, aileron and elevator on one side in under 45 minutes by syringe (VAF). Budget about three days for a whole airframe. (VAF)

Methods

Dispenser method (Tony Kirk)

Built around 3M 08115 Panel Bonding Adhesive. Full parts list with links in the original post: automatic glue dispenser controller, 10cc syringes and air tube, 14-gauge plastic dispenser needles, epoxy gun, spare mixing nozzles.

  • Load only 1-2 cc into a 10cc syringe at a time. The dispenser's own 30cc syringes waste material — pot life is 25-30 minutes.
  • Start the dispense timer at about 0.1 second. Lengthen it a few tenths every 3-4 minutes as the adhesive thickens, reaching about 1.5 seconds near the end of pot life.
  • Use the back-suction setting after each pulse or the adhesive oozes between rivets.
  • Purge the mixing nozzle with a glob before refilling a syringe — the mix cures inside the nozzle.
  • One rivet per trigger click, about one every 2-3 seconds. (F5 preset)
  • Sand flush with the rivet head using a long sanding block and 120 grit. The filler must be painted over. (VAF)

Syringe method (cheaper, no controller)

  • A 20 mL syringe gives comparable results at lower cost and almost no setup. (VAF)
  • SuperFil with a splash of acetone dispenses well; a #10 needle fits the mandrel hole. (VAF)
  • An 18-gauge needle is slow and hard on the hands — one builder developed hand pain over a single wing, and thinning with alcohol did not help. Press the needle out of its adaptor in a bench vise and dispense through the adaptor's own bore, which is close to rivet-hole diameter, using unthinned SuperFil. (VAF)
  • Trim excess with a razor before it hardens, or knock it down later while scuffing for paint.
  • Smooth after cure with Scotch-Brite wheels in a rotary tool (VAF), or sand the heads with 220 grit on a 1x2 block with the skins taped off — about 45 minutes for both sides of a surface, with an emery board for touch-up. (VAF)

Surface prep

No one has reported a filler failure, but nobody has tested it either. Reported practice:

  • Acetone wipe and compressed air only — Sling factory builders, polyester filler, no pop-outs. (VAF)
  • Solvent wipe only, fast-setting body filler, no failures in the following decade. (VAF)
  • Alodine and prime the surface, lightly scuff the rivet heads, then fill with a lightweight filler that sands easily. (VAF)

Fillers builders have used

  • 3M 08115 Panel Bonding Adhesive — the dispenser method is built around it. Sets hard; keep it off anything you do not want it on.
  • SuperFil (Poly-Fiber) — lightweight two-part epoxy filler. Easier to apply than mixing an epoxy micro-balloon slurry, and cures softer than straight epoxy so it feathers cleanly with 150-220 grit. Thin with a splash of acetone to dispense; sets fast in a dispenser, so expect only a couple of cc before it stiffens. (VAF) Part A separates if left on a shelf — invert the container occasionally and stir well before use.
  • JB Weld — readily available. Fill, wipe excess with acetone. Reported holding up over 20 years.
  • Proseal — also 20+ years of service reported. One builder installs the rivets wet, smears Proseal into the hole, then wipes with MEK or acetone.
  • Polyester body filler — what the Sling factory used, applied with a stick.
  • Rage Gold / Rage Extreme — similar automotive body fillers some builders use.
  • Epoxy micro-balloon slurry — stronger, but harder to sand and requires careful mixing.
  • Straight epoxy — not recommended for rivet holes; very hard to sand flush.
  • Poly-Fiber epoxy primer — does not work. Fills well but shrinks and leaves bubble voids. (VAF)

Sources

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