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Rocker/sill internal inspection (access points & warning signs)

Inspect Mazda MX-5 NB rocker/sill cavities for inside-out corrosion warning signs, without damaging drains or creating new moisture traps.

Difficulty
Intermediate
★★★☆☆
Est. Time
60-150 min
Models
NB1 & NB2
Last Updated
2026-03-14

Before You Start / Safety

This is an inspection guide, not a structural rust repair guide. If you find perforation, soft metal, or major seam swelling, escalate to a body shop experienced with MX-5 sill reconstruction.

Warning: “Cavity wax should not be applied to exhaust systems as it will produce smoke and can be flammable.” https://supportdesk.kbs-coatings.com/255826-Cavity-Wax-Provides-Rust-Protection-in-Hard-to-Reach-Areas

Warning: “Spray a continuous coat and do not spray on parts of the brake, engine or exhaust system.” https://irl.sika.com/en/industry/automotive-aftermarket/protective-coatings/cavity-waxes/sikagard-6220.html

Required Tools

  • Bright inspection lamp
  • Non-marring trim tools
  • Flexible borescope/endoscope (recommended)
  • Nitrile gloves + eye protection
  • Non-sharp flexible drain probe
  • Camera/phone for documented baseline photos

Required Parts / Fluids

Inspection-only:

  • No parts required

If prevention is needed after inspection:

  • Cavity wax product and appropriate lance/wand

Model-specific notes (NB1 vs NB2)

  • NB1 and NB2 both suffer from internal rocker/sill moisture retention when drains clog or previous repairs trap water.
  • Exact access points can vary with trim, previous welding, and aftermarket rustproofing.

Manual-reference quote: “1999-2001 Mazda Miata Service Repair Manual” and “2005 Mazda Miata Service Repair Manual” are listed in the Miata factory manual index. https://www.mellens.net/mazda/

Step-by-Step Procedure

1) Confirm moisture path before judging rust severity

Start by checking whether water can exit sill/rocker drain paths. Standing water drives inside-out corrosion.

Community quote: “The problem with these cars is … water pools INSIDE the rockers.” https://www.reddit.com/r/Miata/comments/1od5p1b/.json

2) Inspect external warning signs around rocker/sill and rear drain-hole zones

Look for:

  • paint bubbling near lower sill seams,
  • rust staining around drain-hole areas,
  • swelling at seam overlaps,
  • prior repair transitions (filler, uneven seam line, mismatched texture).

Community confirmation quote: “if it’s bubbling there then the rust is internal” https://www.reddit.com/r/Miata/comments/1qrerkl/.json

3) Use internal viewing where possible (borescope through existing openings)

Prefer existing access points first. Avoid drilling unless you have a defined corrosion-prevention plan and correct sealing/grommeting method.

4) Classify findings conservatively

  • Early risk: surface staining + no seam distortion
  • Likely internal progression: bubbling and seam lift
  • Structural concern: perforation/soft zones/crunchy seam sections

Community quote: “NB’s, like NA’s rust from the inside out.” https://www.reddit.com/r/Miata/comments/1c11wdv/.json

5) If applying cavity wax afterward, protect drain function

Do not flood drain exits shut during corrosion-prevention work.

High-risk quote: “When applying inside cavities, make sure that all drain holes remain open.” https://supportdesk.kbs-coatings.com/255826-Cavity-Wax-Provides-Rust-Protection-in-Hard-to-Reach-Areas

High-risk quote: “In accordance with nearly all OEMs, 3M recommends applying at least three coats … to all unprotected metal surfaces.” https://www.3m.com/3M/en_US/collision-repair-us/featured-products/cavity-wax-plus/

6) Re-test drainage and document a baseline

After any preventive treatment, perform a gentle water-flow check and verify drainage under the car. Save photos/notes for repeat comparison every season.

Torque Specs / Capacities

  • Not applicable for inspection-only tasks.
  • If interior trims, shields, or brackets are removed for access, use year/VIN FSM torque values.

Verification / Post-service checks

  • Drain exits are visibly open and flowing
  • No fresh pooling signs after rain/wash
  • No newly spread bubbling around sill seams after follow-up checks
  • Clear photo baseline saved for seasonal comparison

Uncertainty / Open Questions

  • Accessible public sources strongly align on inside-out corrosion risk and drain-path relevance, but exact NB access-point maps vary by previous repairs and market history.
  • Several general rust-inspection pages were blocked/redirected during this run, so this guide prioritizes retrievable technical references plus direct MX-5 community evidence.

Images

No reusable, clearly licensed NB-specific rocker/sill internal inspection images were added in this run.

Image Credits

  • No images used.

Sources

  1. Mellens.net — Mazda Miata Factory Service Manuals. Retrieved 2026-03-14. https://www.mellens.net/mazda/
  2. 3M — Cavity Wax Plus & Wand Kit For Collision Repair. Retrieved 2026-03-14. https://www.3m.com/3M/en_US/collision-repair-us/featured-products/cavity-wax-plus/
  3. Sika — Sikagard®-6220. Retrieved 2026-03-14. https://irl.sika.com/en/industry/automotive-aftermarket/protective-coatings/cavity-waxes/sikagard-6220.html
  4. KBS Coatings — Cavity Wax Provides Rust Protection in Hard-to-Reach Areas. Retrieved 2026-03-14. https://supportdesk.kbs-coatings.com/255826-Cavity-Wax-Provides-Rust-Protection-in-Hard-to-Reach-Areas
  5. Reddit r/Miata — NB rust on sill - from inside or surface rust? (JSON endpoint). Retrieved 2026-03-14. https://www.reddit.com/r/Miata/comments/1qrerkl/.json
  6. Reddit r/Miata — Prevent Rust NB Miata? (JSON endpoint). Retrieved 2026-03-14. https://www.reddit.com/r/Miata/comments/1od5p1b/.json
  7. Reddit r/Miata — Buying NB and the dreaded R word (JSON endpoint). Retrieved 2026-03-14. https://www.reddit.com/r/Miata/comments/1c11wdv/.json