Different treatment solutions for bare metal, old paint surfaces, and plastic parts (treatment – primer – surfacer comparison)

In the same repair workshop, you may simultaneously face three types of substrates: bare metal, old paint surface, plastic parts. Their "temperaments" are completely different: bare metal fears rust, old paint surface fears instability, plastic parts fear poor adhesion. Using the same process to treat them will inevitably cause problems. This article provides a "treatment—primer—surfacer" combination plan based on substrate to help you scientifically select the process.

In one sentence: The substrate determines the process—bare metal requires rust prevention first then epoxy primer, old paint surfaces need stability assessment before feathering, plastic parts must use PP adhesion promoter as a bridge; these three schemes must not be mixed.

I. Bare Metal

Bare metal begins to oxidize within hours of exposure to air and is the most "urgent" substrate.

• Treatment: Degrease → Polish to bright metal (80–180 grit) → Apply immediately.
• Primer: Epoxy primer (preferred for rust prevention) or etch primer.
• Middle coat: Depending on the situation, apply the middle coat directly or apply putty followed by the middle coat.

II. Old paint film (intact / loose)

The old paint surface should first be assessed for stability: use cross-cutting or sanding to test; if it is firm, keep it, but if it is loose/blistered, it must be removed.

• Treatment (firm): Degreasing → Sanding and feathering (to provide roughness).
• Treatment (soft/loose): Sand to a sound substrate; if necessary, expose the metal and then follow the bare metal procedure.
• Primer / Surfacer: Usually applied directly with surfacer without primer, then topcoat.

III. Plastic Parts

See the dedicated section on plastic parts; the core is release agent removal + PP water.

• Treatment: Identify material → Remove release agent → (PP water / flame / plasma).
• Primer: PP water/plastic primer, not metal epoxy primer.
• Mid-coat: Flexible mid-coat, matching the elasticity of plastic.

Application key points: For bare metal, apply anti-rust primer "as soon as possible"; for old paint surfaces, "assess adhesion first" before deciding to retain or remove; for plastic parts, "better to use PP adhesion promoter" rather than skip it. Mixing the three solutions is a high-risk zone for coating defects.

Summary

Substrate determines the process route: Bare metal focuses on rust prevention (epoxy primer, the faster the better), Old paint surface focuses on assessing stability (if firm, feather and retain; if loose, remove), Plastic parts focus on adhesion (PP water as a bridge + flexible surfacer). Pairing "preparation—primer—surfacer" according to the substrate can avoid rework such as paint peeling and bubbling.

Frequently Asked Questions (FAQ)

Why should bare metal be primed as soon as possible?

Exposed bare metal will rapidly oxidize and rust, potentially affecting adhesion within a few hours. After sanding to bright metal, epoxy or etch primer should be applied as soon as possible to form an anti-rust barrier.

Does the old paint surface have to be completely sanded off?

Not necessarily. First assess its stability: sound old paint can be kept after feather sanding and directly receive the mid-coat; only blistered, cracked, or soft parts need to be removed down to a sound layer or bare metal.

Can plastic parts use epoxy primer?

Generally not recommended to use metal epoxy primer directly. PP-type plastics should use PP adhesion promoter/plastic primer to build an adhesion bridge; flexible surfacer also better matches the expansion and contraction of plastic, reducing the risk of cracking.

How to determine whether the old paint surface is sound?

You can use the cross-cut method, sanding test, or observe for blistering/chalking. If a powdery loose layer or whole-sheet peeling appears during sanding, it is unsound and must be removed; if sanding yields a uniformly smooth surface with good adhesion, it can be kept.

Can the three substrates share one mid-coat system?

Not entirely. Metal parts use conventional mid-coat; plastic parts are recommended to use flexible surfacer to match their elasticity; if bare metal has been sealed with epoxy primer, the mid-coat choice is relatively free. The key is that primer-substrate compatibility takes priority.

Further Reading

• Differences Between Water-Based and Solvent-Based Paint
• The Function and Types of Automotive Primer
• Basic Process of Automotive Painting
• Analysis of Automotive Paint Coating Structure

Kexin New Materials · Automotive Coating Technology Library | This article is technical Q&A content, for reference in selection and application

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