A pinhole-sized pore on the paint film, or even blistering—this is solvent/gas that "tried to escape but couldn't" and got trapped in the coating.
In a nutshell: Pinholes and bubbles = underlying gas/solvent trapped after surface drying; solve by spraying after fully dry, using matched thinner, and raising temperature slowly.
I. Causes
Undercoat not fully dry: Solvents or moisture in the putty/primer are trapped after the top coat is sprayed, and are pushed out when heated.
Thinner mismatch: Volatilizes too fast or in excess, gas has no time to escape.
Too rapid temperature rise: Surface dries too quickly forming a skin, and internal gas breaks through to form pinholes.
Figure: Solvent/gas at the bottom layer is sealed after surface drying, and is pushed out to form pinhole bubbles.
II. Solutions
Thoroughly dry: Allow the putty/primer to dry completely before applying the topcoat.
Matching diluent: Select a diluent with an appropriate evaporation rate according to the temperature zone.
Slow heating: Allow sufficient flash-off before baking to avoid rapid temperature rise.
Figure: Dry thoroughly + matching thinner + slow temperature rise eliminates pinholes at the source.
Remedy: Existing pinholes must be sanded to a clean layer, then primer/basecoat applied and repainted; severe bubbles should be scraped off and redone, and the uncured underlying layer identified.
Summary
Pinholes and bubbles occur when the underlying filler/primer is not fully dry, the thinner is incompatible, or the temperature rises too quickly, causing solvent vapor to be trapped and forced out after surface drying. The solution is to ensure adequate drying, use a compatible thinner, and raise the temperature gradually; if already present, sand and repaint, and investigate the root cause.
Frequently Asked Questions (FAQ)
What causes pinholes in spraying?
Mainly it is because the underlying putty or primer is not fully dry, the thinner is incompatible causing abnormal evaporation, or heating too fast causes surface skinning, trapping internal solvent gases which are forced out to form pinholes.
Are bubbles and pinholes the same thing?
They share the same origin but differ in appearance: a small amount of gas forced out creates tiny holes, i.e. pinholes; a larger amount of gas forms raised blisters, i.e. bubbles. In essence, both are caused by underlying gas/solvent being sealed by the surface layer.
How to solve pinholes?
Let the underlayer dry thoroughly before spraying the topcoat, choose a thinner with a matching evaporation rate according to the application temperature, allow sufficient flash-off before baking, and avoid rapid temperature rise.
How to repair existing pinholes?
Sand down to a clean layer, apply primer/basecoat and respray; for severe bubbles, scrape off and redo, and find out which layer was not fully dry.
Will using the wrong thinner cause pinholes?
Yes. A thinner that evaporates too fast causes surface skinning before the gas escapes, easily leading to pinholes; excessive dilution also increases the gas volume.
Kexin New Materials · Automotive Coating Technology Library | This article is technical Q&A content, for reference in selection and application
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