CSLB C-33 Painting & DecoratingCoating ApplicationMedium
A painting crew is applying a high-solids, solvent-borne epoxy primer to a concrete floor at a specification of 10 mils dry film thickness (DFT). After application, they notice numerous small, circular craters in the wet film that extend down to the substrate, resembling a 'moon surface'. What is the most likely cause of this defect?
- AApplying the primer in excessively high humidity conditions
- BInsufficient induction time after mixing
- CAir entrapment or outgassing from the porous concrete substrate
- DOver-thinning of the primer
Show answer & explanationAnswer & explanation
Correct answer: C. Air entrapment or outgassing from the porous concrete substrate
The description of small, circular craters extending to the substrate, especially on a concrete floor, is characteristic of 'pinholing' or 'cratering' caused by air entrapment within the porous concrete or outgassing from the substrate as the coating cures. Concrete is notorious for this issue.
Why the other options are wrong
- A. High humidity can cause blushing or slower drying for some coatings, but not typically pinholing down to the substrate.
- B. Insufficient induction time can lead to poor cure, reduced pot life, or poor adhesion, but not typically pinholing.
- D. Over-thinning typically leads to sags, runs, or poor hiding, not craters extending to the substrate.
Pinholing (Concrete Epoxies)
Small, circular craters in a coating film that extend down to the substrate, often caused by trapped air or outgassing from porous substrates like concrete.
- Common in high-solids coatings on concrete.
- Caused by air escaping from the substrate as the coating cures.
- Prevented by proper surface preparation (e.g., shot blasting, filling bug holes) and sometimes a 'scratch coat' or primer.
Memory trick: Moon craters on the floor, air escaping, that's the core.