C33 Surface Preparation for Various Substrates 2 — Questions and Answers
Question 1: Before painting new galvanized steel, what surface preparation issue must a C-33 contractor address?
- The surface is too rough for paint
- Zinc oxide and oils from the galvanizing process prevent paint adhesion (Correct answer)
- Galvanized steel cannot be painted
- The surface must be sand-blasted to bare steel
Correct answer: Zinc oxide and oils from the galvanizing process prevent paint adhesion
New galvanized steel has a zinc oxide layer and manufacturing oils that repel standard paints. The surface must be cleaned and treated to promote adhesion.
New galvanized steel presents adhesion challenges due to zinc oxide formation and manufacturing oils. Preparation options include: solvent cleaning plus mechanical abrasion, acid etching with phosphoric acid solutions, or applying a galvanized metal primer with modified resins that bond to zinc.
Question 2: What is the recommended method for testing surface moisture content in concrete before painting?
- Visual observation of color
- ASTM D4263 plastic sheet test or electronic moisture meter (Correct answer)
- Touching the surface with bare hands
- Waiting a standard number of days after pouring
Correct answer: ASTM D4263 plastic sheet test or electronic moisture meter
The ASTM D4263 plastic sheet test and electronic moisture meters are the standard methods for verifying concrete moisture content before painting.
Two primary methods: (1) ASTM D4263 plastic sheet test - tape plastic to the surface for 16+ hours and check for moisture underneath. (2) Electronic moisture meters provide immediate readings; concrete should be below 4-5% moisture. Concrete typically requires 28+ days of curing before painting.
Question 3: When preparing previously painted exterior wood for repainting, what level of surface preparation is typically required?
- Complete paint removal to bare wood
- Remove only loose, flaking paint; feather edges and sand glossy areas (Correct answer)
- No preparation if the existing paint is in good condition
- Apply new paint directly over any existing condition
Correct answer: Remove only loose, flaking paint; feather edges and sand glossy areas
Standard preparation involves removing loose, flaking paint by scraping and sanding, feathering rough edges, and sanding glossy surfaces for adhesion.
Exterior wood repainting preparation: (1) Scrape all loose, flaking paint, (2) Sand to feather scraped edges, (3) Sand glossy surfaces to create adhesion, (4) Clean with detergent, (5) Rinse and dry, (6) Caulk and fill, (7) Prime bare spots. For pre-1978 buildings, test for lead before disturbing paint.
Question 4: What is the purpose of applying an 'etch primer' to smooth, non-porous surfaces like aluminum or fiberglass?
- To add color to the surface
- To chemically etch the surface and create microscopic adhesion points for the topcoat (Correct answer)
- To make the surface waterproof
- To fill large surface imperfections
Correct answer: To chemically etch the surface and create microscopic adhesion points for the topcoat
Etch primers contain mild acids that chemically react with the substrate, creating microscopic texture and chemical bonds that improve paint adhesion.
Etch primers contain phosphoric acid that creates a conversion coating of metal phosphate crystals on the surface. This provides both chemical adhesion and microscopic mechanical tooth. Etch primers are typically very thin (0.3-0.5 mils DFT) and serve as an adhesion promoter, not a full primer.
Question 5: A C-33 contractor is preparing drywall with multiple joint compound patches. What preparation ensures a uniform paint finish?
- Apply paint directly - patches will blend in
- Prime the entire surface with a PVA drywall sealer to equalize absorption rates (Correct answer)
- Only prime the patched areas
- Sand the patches flush and paint without primer
Correct answer: Prime the entire surface with a PVA drywall sealer to equalize absorption rates
Joint compound and drywall paper have different porosity. Priming the entire surface equalizes absorption, preventing 'flashing' at patch locations.
Drywall paper and joint compound have different porosity. Without proper priming, compound areas absorb more paint, creating visible flashing. A PVA primer-sealer applied over the entire surface equalizes porosity. Spot-priming patches only can worsen flashing by creating three different absorption zones.
Question 6: What does 'profile' mean in the context of surface preparation for industrial painting on steel?
- The cross-section shape of the steel beam
- The microscopic peak-to-valley roughness pattern created by abrasive blasting (Correct answer)
- The chemical composition of the steel alloy
- The thickness of the existing paint system
Correct answer: The microscopic peak-to-valley roughness pattern created by abrasive blasting
Surface profile is the microscopic roughness pattern created by abrasive blasting, measured in mils. It provides the mechanical tooth for coating adhesion.
Surface profile is the peak-to-valley height of the roughness pattern, typically 1.0-4.0 mils. The coating fills the valleys and locks onto the peaks. Profile is measured using replica tape or stylus gauges. Too shallow means inadequate adhesion; too deep means peaks may not be covered, creating rust points.
C33 Surface Preparation for Various Substrates 2
Before painting new galvanized steel, what surface preparation issue must a C-33 contractor address?