The paint surface has bulged a small bubble, and when punctured, there is rust underneath—this is the most headache-inducing rust recurrence and blistering. It is rarely a "paint problem"; it is almost always a substrate preparation problem. This article clarifies the root causes and prevention.
In one sentence:Bubbling and rust recurrence = moisture intruding through defects + substrate not truly rust-proofed; the root cause lies in "pretreatment," not the topcoat.
I. Causes of blistering and rust bleeding
Insufficient substrate preparation: Residual rust, oil, or old paint leads to poor film adhesion, allowing moisture to seep in.
Bare metal exposure: After grinding to expose the steel, the epoxy primer was not applied in time, resulting in immediate flash rusting.
Coating has pores: Pinholes/scratches allow water and oxygen to reach the steel plate.
Environmental moisture: High-humidity construction, uncured water grinding, moisture trapped between layers.
Figure: Moisture intrudes at the defect, the substrate rusts and expands, lifting the paint film to form rust bubbles.
II. Fundamental Prevention and Control
Thorough pretreatment: Degreasing → Derusting → Sanding → Dust removal, immediately seal bare iron with epoxy primer.
Sealed defect: Use primer to fill the holes to prevent pinholes from penetrating directly to the steel plate.
Controlled environment: Construct under low humidity, and ensure thorough drying after water grinding.
Figure: Standard pretreatment + epoxy primer sealing blocks rust recurrence at the source.
What to do if blistering has already occurred: Scrape off the blisters, sand down to healthy substrate, remove rust, then redo epoxy primer → intermediate coat → topcoat. Never just patch paint on the surface to "cover it up", otherwise it will blister again nearby.
Summary
Blistering and rust rebound mostly result from inadequate substrate preparation, unprotected bare iron, or coating porosity that allows moisture ingress, causing steel plate corrosion and expansion. The fundamental prevention lies in standardized pretreatment and epoxy primer sealing; existing blisters must be scraped down to sound substrate and the system redone—surface covering is not acceptable.
Frequently Asked Questions (FAQ)
Why do rust bubbles appear on the paint surface?
Mostly due to inadequate substrate preparation (residual rust/oil/old paint), bare iron not sealed in time, or pinholes in the coating, allowing moisture and oxygen to reach the steel plate and cause corrosion expansion that lifts the paint film and forms bubbles.
Is bubbling and rust bleed-through caused by bad paint?
Mostly not. The root cause lies in substrate pre-treatment—if rust is not truly removed and epoxy primer is not applied for sealing, no matter how good the topcoat is, it cannot block moisture from intruding through defects.
How to fundamentally prevent rust bleed-through?
Thorough pre-treatment (degreasing, derusting, sanding, dust removal), immediately seal bare iron with epoxy primer, fill and level pinhole defects, and apply in low-humidity conditions, with full drying after wet sanding.
How to repair if bubbles have already appeared?
Scrape off the bubbles, sand to healthy substrate, remove rust, then redo epoxy primer → intermediate coat → topcoat. Merely covering with surface paint will cause bubbling elsewhere again.
Why is wet sanding prone to rust bleed-through?
If not thoroughly dried after wet sanding, moisture gets sealed between layers by subsequent coatings and rust expands into bubbles upon contact with oxygen. Therefore, after wet sanding it must be completely dried before application.
Kexin New Materials · Automotive Coating Technology Library | This article is technical Q&A content, for reference in selection and application
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