I'm bidding an interior repaint with a lot of drywall repairs.

Spot-priming (hitting only patches) is standard for small repairs on previously painted, sound walls. Full-wall priming is required when the substrate is inconsistent, porous, or compromised.

Spot-prime when:

• Patches are small (nail holes, screw pops, minor cracks).
• Existing paint is intact, clean, and uniformly sheened (e.g., eggshell/satin).
• Topcoat is similar sheen/color to existing.
• Method: Use a high-hide, stain-blocking primer (shellac or water-based PVA) only on the mud. Feather edges. Sand smooth.

Full-wall prime when (IRC R702.7 / ASTM C840 / PDCA P5):

• New bare drywall: Mud and paper have wildly different porosity. Without a full PVA/sealer coat, you get "flashing" (sheen/color variation) on the finish.
• Skim coats / Level 5 finish: The entire surface is new compound.
• Drastic color change (dark to light) or sheen jump (flat to semi-gloss).
• Stains/odors: Water, smoke, tannin, grease (shellac or oil-based primer full coat).
• Chalking/peeling paint: Requires bonding primer (e.g., Zinsser Peel Stop) over whole area.
• Glossy existing paint: Requires scuff-sand + bonding primer for adhesion.

The "Flashing" Test: After spot-priming and topcoating, shine a work light parallel to the wall at 45°. If patches glow dull or shiny vs. the field, you needed full prime.

Bid Smart: Price spot-prime as "prep." Price full prime as a separate line item ("Prime entire walls: 1 coat PVA/Stainblocker"). Explain why (warranty, uniformity). If the GC/homeowner pushes back, document the flashing risk in writing.

Bottom line: Spot-prime repairs; prime the system when the substrate isn't uniform.

Spot-priming patches versus priming the entire wall comes down to uniform sheen, adhesion, and long-term appearance — especially critical in interior repaints with extensive drywall repairs.

Spot-priming patches only means applying primer just over repaired areas (e.g., mudded seams, nail pops, patches). This seals the porous joint compound and prevents “flashing” — where the patch absorbs more topcoat, appearing duller or shinier than the surrounding wall. It’s sufficient when:

• The existing wall is in good, uniform condition (same sheen, no stains, no prior patching history).
• You’re using a high-quality, hide-rich topcoat (e.g., premium acrylic latex) over a similar sheen (e.g., eggshell to eggshell).
• The substrate is clean, sound, and not glossy (which would require bonding primer).

Priming the entire wall is necessary when:

• The existing wall has inconsistent sheen (e.g., semi-gloss trim paint on walls, or patchy old paint).
• There are stains (water, smoke, grease) that could bleed through.
• The wall is bare drywall or has extensive patching (>20–30% surface area).
• You’re making a dramatic color change (dark to light or vice versa).
• The existing paint is chalky, peeling, or low-quality (poor adhesion risk).
• You’re using a low-hide topcoat (e.g., deep colors, some flat finishes).

Code/Standard Reference: While not a direct code issue, ASTM D4263 (moisture testing) and ASTM D4258 (surface cleaning) imply proper surface prep. Manufacturers (e.g., Sherwin-Williams, Benjamin Moore) require full priming over porous or repaired substrates to ensure warranty coverage and prevent premature failure.

Practical Advice: For bidding, if repairs exceed 10–15% of the wall surface or the wall has variable condition, prime the whole wall. It’s faster than masking and spot-priming dozens of patches, ensures uniform topcoat absorption, and eliminates callbacks for flashing or poor adhesion. Use a PVA drywall primer for new repairs or a multi-surface primer (e.g., Zinsser Bulls Eye 1-2-3) for existing painted surfaces.

Bottom line: Spot-priming saves time and material only on walls in near-perfect condition. Otherwise, full priming is the professional, durable choice.