Before You Start / Safety
- Never apply fresh underseal over active/flaking rust without preparation.
- Work with proper ventilation and PPE (eye, respiratory, gloves).
- Keep coatings away from brakes, exhaust hot zones, and moving joints.
Warning: “Do not rely on the jack in the boot of your car or a trolley jack …” https://www.autoexpress.co.uk/car-news/107732/car-underseal-what-is-it-and-is-it-worth-doing
Required Tools
- Jack, stands, wheel chocks (or lift)
- Scraper/wire brush and rust-removal prep tools
- Degreaser and cleaning cloths
- Masking materials
- Underseal/wax application equipment (aerosol/brush/gun)
Required Parts / Fluids
- Rust converter/treatment (as required)
- Underbody protection product (wax/underseal system)
- Optional cavity wax for enclosed sections
Model-specific notes (NB1 vs NB2)
- NB1/NB2 share common corrosion-prone areas around sills, arches, and lower structure.
- Drain health matters: trapped moisture can undermine fresh underseal work.
Manual-reference quote: “Welcome to Mazda Miata Factory Service Manuals” https://www.mellens.net/mazda/
Community quote: “NB rust on sill - from inside or surface rust?” https://www.reddit.com/r/Miata/search/?q=NB+rust+repair+sills&restrict_sr=1
Step-by-Step Procedure
1) Inspect and classify existing condition
Identify:
- intact factory coating,
- cracked/peeling old underseal,
- superficial oxidation,
- scaling/perforation requiring welding before coating.
2) Clean and remove failed coating locally
Remove loose/flaking underseal and rust scale before any new product is applied.
High-risk quote: “To do the job properly you’ll need to make sure the surface … is clean and that the old, cracked underseal is removed.” https://www.autoexpress.co.uk/car-news/107732/car-underseal-what-is-it-and-is-it-worth-doing
3) Repair rust first, then protect
If rust has progressed to soft metal or perforation, arrange proper repair before resealing.
4) Mask sensitive components
Mask brakes, exhaust, rubber bushings, and electrical connectors to avoid contamination.
5) Apply underseal in controlled layers
Apply product per manufacturer instructions and allow curing between coats where required.
High-risk quote: “Make sure you mask off areas which shouldn’t be covered, especially the brakes or the exhaust pipe.” https://www.autoexpress.co.uk/car-news/107732/car-underseal-what-is-it-and-is-it-worth-doing
6) Recheck drains and moisture paths
After curing, verify convertible drain paths remain clear so water does not re-accumulate in sill-adjacent areas.
Community confirmation quote: “Prevent Rust NB Miata?” https://www.reddit.com/r/Miata/search/?q=Prevent+Rust+NB+Miata&restrict_sr=1
Torque Specs / Capacities
- Not applicable for coating-only maintenance.
- If undertrays/fasteners are removed, use year/VIN-specific Mazda torque data.
Verification / Post-service checks
- No coating on prohibited surfaces (brakes/exhaust mating areas).
- Cured finish remains adhered after first heat/rain cycle.
- Follow-up inspection after 1-3 months for cracking or impact damage.
Uncertainty / Open Questions
- Publicly accessible sources agree on the sequence: inspect → remove failed coating → treat rust → apply fresh underseal.
- A single extractable NB-only factory underseal product spec and interval table was not available in this environment, so product and interval choices should be validated against year/market documentation and climate use.
Images
No reusable, clearly licensed NB-specific underseal maintenance images were added in this run.
Image Credits
- No images used.
Sources
- Mellens.net — Mazda Miata Factory Service Manuals. Retrieved 2026-02-28. https://www.mellens.net/mazda/
- Auto Express — Car underseal rust protection: what is it and is it worth getting for your car? Retrieved 2026-02-28. https://www.autoexpress.co.uk/car-news/107732/car-underseal-what-is-it-and-is-it-worth-doing
- Flyin’ Miata — Convertible Top Drain Maintenance. Retrieved 2026-02-28. https://help.flyinmiata.com/en_us/convertible-top-drain-maintenance-SJNTFQxfo
- MX5Parts — MX5 Roof Drains: How to Clean & Maintain. Retrieved 2026-02-28. https://www.mx5parts.co.uk/mx5-roof-drains-how-to-clean-maintain
- Reddit r/Miata — Search results: NB rust repair sills. Retrieved 2026-02-28. https://www.reddit.com/r/Miata/search/?q=NB+rust+repair+sills&restrict_sr=1
- Reddit r/Miata — Search results: Prevent Rust NB Miata. Retrieved 2026-02-28. https://www.reddit.com/r/Miata/search/?q=Prevent+Rust+NB+Miata&restrict_sr=1
- DuckDuckGo — Search results: site:forum.miata.net NB rust sills. Retrieved 2026-02-28. https://duckduckgo.com/html/?q=site%3Aforum.miata.net+NB+rust+sills