I'm painting factory-primed fiber-cement lap siding on a new house.

Yes, fiber-cement behaves differently than wood. Key differences:

Surface prep is critical. Factory primer is often slick and chalky. You must wash it (TSP substitute, rinse well) and lightly sand (120-grit) to de-gloss and remove chalk. Skip this, paint peels in sheets.

Caulk differently. Don't caulk the horizontal lap joints (the "shadow line"). Fiber-cement expands/contracts less than wood but still moves; caulking traps moisture and causes paint failure. Only caulk vertical joints, corners, and around penetrations. Use high-quality polyurethane or STPE sealant (not silicone—unpaintable).

Back-prime cut ends. Any field-cut edge must be primed before installation. Unsealed ends wick water, causing swelling/delamination. Use oil-based or 100% acrylic primer.

Paint specs. Use 100% acrylic latex (satin or flat). Two coats. Minimum 4 mils DFT total. Dark colors (LRV < 20) increase thermal stress; check manufacturer's color restrictions (e.g., James Hardie prohibits certain dark hues).

Fastener heads. Spot-prime nail/screw heads if they breach the factory primer. Don't rely on topcoat alone.

Code/Standards: Follow ASTM C1186 (material) and manufacturer's written instructions (IRC R703.10.1). James Hardie, Allura, and Nichiha have slightly different joint/clearance specs—read the specific brand's guide.

Bottom line: Prep the factory primer aggressively, don't caulk laps, seal every cut end, and follow the brand's install manual.

Painting factory-primed fiber-cement lap siding differs from wood in key ways—follow these practical steps:

1. Surface Prep: - Factory primer is designed for paint adhesion but may have a glossy or waxy finish. Lightly sand with 120–150 grit to degloss (check manufacturer—some prohibit sanding; if so, use a bonding primer). - Clean thoroughly: remove dust, oils, or efflorescence with TSP or a dedicated cleaner. Rinse and dry completely. - Unlike wood, no need to address rot or grain raising, but inspect for cracks or damage—repair with fiber-cement-compatible patch.

1. Primer (If Needed): - If the factory primer is intact and compatible with your topcoat (verify with siding/paint mfr), you may skip priming. - If sanding, recoating after weathering, or using a drastic color change, apply a 100% acrylic bonding primer (e.g., Zinsser Bulls Eye 1-2-3, Sherwin-Williams Loxon). Do not use oil-based—it can trap moisture.

1. Paint Selection: - Use 100% acrylic latex exterior paint (flexible, breathable, UV-resistant). Avoid vinyl-acrylics—they’re less durable on cementitious surfaces. - Dark colors? Choose formulations with IR-reflective technology to reduce heat buildup and warping risk (per James Hardie guidelines).

1. Application: - Brush/roll or spray (back-roll if spraying for even coverage). - Apply two coats—factory primer counts as one; topcoat needs two for full hide and warranty compliance. - Follow temp/humidity specs (typically 50–90°F, <85% RH).

1. Critical Differences vs. Wood: - No priming bare substrate (it’s already primed). - Less prone to swelling/shrinking, but moisture management is still vital—ensure proper flashing and caulking at joints (use paintable, polyurethane-based caulk). - Never stain or use semi-transparent finishes—fiber-cement isn’t designed for it.

Code Note: IRC R703.10.1 requires siding to resist weather; painting maintains this. Always defer to siding manufacturer’s specs (e.g., James Hardie, Nichiha)—warranties often require specific paint systems. If unsure about adhesion or compatibility, consult a paint rep or licensed painter. ≤300 words.