What to Put on Bare Metal on a Car

Apply a 2K direct-to-metal epoxy primer as the first coating on clean, degreased, freshly abraded bare steel or aluminum; then build the finish with body filler (if needed), 2K urethane primer-surfacer, sealer, and topcoat. Use seam sealer on joints, cavity wax inside closed sections, underbody stone-chip/undercoating over epoxy, and weld-through primer only where you’re welding. This sequence prevents corrosion, promotes adhesion, and delivers a durable finish.

Why bare metal needs immediate protection

Automotive metals flash-rust or oxidize quickly, especially in humid or coastal environments. The best defense is a non-porous barrier coat that seals out moisture and salts while anchoring later layers. Modern 2K epoxy primers provide superior corrosion resistance and adhesion compared with older self-etch systems and rattle-can primers, making them the industry standard for restoration and repair.

The gold-standard system for bare steel

The following sequence outlines a widely accepted, durable process from fresh metal to topcoat. It prioritizes corrosion resistance, adhesion, and compatibility. Always read each product’s technical data sheet (TDS) and respect mix ratios, film builds, and recoat windows.

1. Degrease: Use a dedicated wax and grease remover before any sanding. Avoid silicone-containing cleaners.
2. Abrade to bright metal: Sandblast or DA-sand to clean, shiny metal (typically 80-grit for profile). Remove all scale and rust. If pitting remains, mechanically clean thoroughly.
3. Optional metal prep: If using a phosphoric-acid metal conditioner, follow the TDS exactly, rinse/neutralize if required, and ensure no white residue remains. Many epoxies prefer bare, clean, dry metal with no acid residues.
4. Immediate epoxy primer (DTM): Spray two medium-wet coats of a 2K epoxy primer within minutes of prep to prevent flash rust. Observe induction time if specified and proper flash between coats.
5. Body filler as needed: After epoxy flashes/cures to the manufacturer’s window, scuff the epoxy and apply polyester filler or glaze. This protects metal beneath the filler from moisture. Some shops still apply filler to bare metal; epoxy-first is the safer corrosion approach.
6. 2K urethane primer-surfacer: Apply, block-sand, and repeat as necessary to straighten panels. Keep film builds within TDS limits.
7. Sealer: Apply a compatible sealer to lock down sanded primer and ensure uniform topcoat absorption and color holdout.
8. Basecoat/clearcoat or single-stage: Finish with the chosen topcoat system, observing flash, film builds, and booth conditions.

This sequence gives a fully sealed substrate with strong adhesion through the stack, minimizing the risk of corrosion creep and finish defects over time.

Alternatives and special cases

Some areas and scenarios call for different first coats or additional products to protect the metal and ensure serviceability.

• Weld areas: Use a zinc- or copper-rich weld-through primer on mating flanges before welding. After welding, clean thoroughly and apply epoxy primer.
• Inside cavities (rockers, doors, pillars): After painting, flood with cavity wax using wand applicators to protect hidden seams and flanges.
• Seams and overlaps: Over epoxy, apply a paintable 2K polyurethane seam sealer; then topcoat. Do not leave bare seams unsealed.
• Underbody and wheel wells: Epoxy first, then a high-build stone-chip or polyurethane/polyurea coating; avoid asphalt/tar products on modern repairs.
• Temporary protection: If you can’t spray 2K epoxy immediately, use a 2K epoxy aerosol or a zinc-rich primer as a short-term measure, and top with epoxy as soon as possible. Oil/wax films (e.g., lanolin-based) can buy time but must be fully removed before painting.
• Spot repairs: For small chips to bare metal, clean, de-rust, apply epoxy or a compatible DTM primer, then touch-up. Avoid leaving etch primer as the only protection.

Choosing the correct approach for the area’s function—welded, hidden, or exposed—improves both longevity and repairability.

Common mistakes to avoid

These pitfalls often cause premature rust, adhesion loss, or finish defects.

• Waiting too long after sanding: Flash rust can form in minutes; prime promptly.
• Applying filler over self-etch primer: Many systems prohibit this; confirm TDS or apply filler over scuffed epoxy instead.
• Stacking incompatible primers: Do not put self-etch under epoxy unless the epoxy TDS allows it.
• Skipping seam sealer and cavity wax: Unsealed overlaps and cavities trap moisture and rust from the inside out.
• Using lacquer or 1K primers as the base: They are porous and not corrosion-barrier coats.
• Contaminants on metal: Fingerprints, silicone, or brake cleaner residue can cause fisheyes and adhesion issues.

A clean, compatible, and timely process is crucial to long-term success.

Conditions, safety, and tools

Correct environment and PPE are essential for quality and health, especially with isocyanate-containing 2K products.

• Environment: 18–29°C (65–85°F), humidity controlled, and substrate at least 10°F (6°C) above dew point.
• Ventilation: Use a spray booth or well-ventilated area with proper filtration to control overspray and dust.
• PPE: Supplied-air respirator for isocyanate-containing 2K products; gloves, eye protection, paint suit.
• Tools: DA sander (80–180 grit), scuff pads, tack cloths, quality HVLP gun, mixing cups, and a calibrated scale when required.

Following environmental specs and PPE guidance in each product’s TDS protects both the finish and the technician.

Product compatibility and timelines

Flash and recoat windows

Respect the epoxy primer’s minimum and maximum recoat times. Applying filler or primer-surfacer too soon can trap solvents; waiting too long may require sanding or re-activation for chemical adhesion.

Filler over or under epoxy?

Modern best practice is epoxy-first, then filler over scuffed epoxy for corrosion protection beneath the filler. If you apply filler on bare metal, cover it with epoxy afterward to seal it.

If you can’t spray 2K

Use a 2K epoxy aerosol with internal hardener as a short-term barrier; later, sand and proceed with a full system. Avoid relying on 1K primers as the only protection layer.

Quick reference for other metals

Different substrates need tailored prep to ensure adhesion and avoid galvanic issues.

• Aluminum: Degrease, avoid steel cross-contamination, abrade with fresh paper or Scotch-Brite, then epoxy primer or an approved acid-etch/wash primer followed by epoxy. Non-chrome conversion coatings may improve adhesion and corrosion resistance.
• Galvanized steel: Thoroughly degrease, lightly abrade to de-gloss without cutting through zinc, then use an epoxy approved for galvanized substrates or a manufacturer-specified wash primer before epoxy.
• Stainless: Degrease, abrade, and use an epoxy designed for stainless; ensure thorough scuffing for mechanical key.
• Weld seams and HAZ: Remove oxides after welding, clean, and epoxy promptly; apply seam sealer over epoxy where appropriate.

Matching the primer system to the substrate prevents adhesion failures and underfilm corrosion.

Maintenance and long-term protection

After refinishing, simple maintenance extends the life of your work.

• Wash and decontaminate: Remove road salts and iron fallout periodically.
• Inspect chip-prone areas: Touch up stone chips quickly to prevent corrosion creep.
• Renew cavity wax: Reapply every few years in harsh climates or after invasive repairs.
• Underbody checks: Inspect coatings for impact damage and reseal as needed.

Routine inspection and quick touch-ups prevent small breaches from becoming major repairs.

Summary

For bare automotive metal, the proven recipe is immediate 2K DTM epoxy primer on clean, abraded metal, followed by filler (if needed), 2K urethane primer-surfacer, sealer, and topcoat. Augment with seam sealer, cavity wax, and appropriate underbody coatings, and use weld-through primer only where welding. Adhering to product TDS, environmental controls, and safety protocols delivers a durable, corrosion-resistant finish.

What to put on a car bare metal?

How to Prepare a Bare Metal Surface for Automotive Painting

1. Wash the auto body part with surface cleaner.
2. Apply some metal conditioner evenly on the surface.
3. Spray the surface with DTM primer.
4. Spray a second coat of primer.
5. Roughly sand the surface using P320 sandpaper.
6. Finish by hand.

What to put on bare metal so it doesn’t rust?

At the least, you should use several coats of polyurethane.

Should you put body fillers over bare metal?

A common misconception is that you can’t apply filler over bare metal because it won’t stick, which is why many use epoxy first. But here’s the truth: if the metal is prepped correctly by aggressively scratching the surface with 24 or 36 grit sandpaper, there’s no way the filler will come off.

Can you paint over bare metal on a car?

The best paint for bare clean metal is a two part epoxy. You can do body work or repairs on top of that coat – and it will not rust. You then paint or top coat after all is good and straight. Etch primer is for spot priming repairs.