Exterior Repaint Guide for Contractors: Substrates, Prep, and Coating Systems

An exterior repaint is never just a paint job. It is a substrate rehabilitation project where the coating is the last step, not the first. Contractors who treat exterior work as a straight repaint — clean, prime, paint — consistently produce callbacks, warranty claims, and margin erosion. This guide covers the full professional workflow: how to assess substrate condition by material type, what preparation each substrate demands, how to select the right coating system, and how to estimate exterior work accurately using PCA P10 production rates.

Substrate Assessment: The Pre-Quote Walkthrough

Never bid an exterior repaint without a systematic walkthrough. Budget an hour for a standard residential repaint; more for complex elevations. The goal is to identify every condition that affects prep scope, product selection, and schedule — before you price the job.

Assessment checklist by substrate type:

Moisture testing: On wood substrates, always test moisture content before pricing prep. A reading above 15%MC means the wood cannot be coated until it dries — if that will delay the project schedule, this needs to be in your scope notes. See our moisture testing guide for the correct procedure using pin-type meters at multiple depths.

Paint film integrity test: Tape test per ASTM D3359. Press a 2-inch piece of high-tack tape firmly onto the existing coating and remove sharply at 90°. Significant paint removal means adhesion is compromised — either from multiple coating layers or from substrate moisture — and the spec must address this before recoating.

Surface Preparation by Substrate

Wood Siding

1. Power wash at 1,500–2,500 PSI with a 25° or 40° fan tip — hold perpendicular to the board, not at an angle that drives water under laps
2. Allow full drying — minimum 48–72 hours in normal conditions, longer for dense species and north-facing elevations
3. Scrape all loose and peeling paint back to a firm edge; feather by hand sanding (80 grit) to reduce hard edges
4. Spot prime all bare wood immediately — bare wood weathers rapidly and should never be left exposed overnight
5. Fill checking cracks and open lap joints with an appropriate paintable exterior caulk or wood filler before priming
6. Sand all repaired areas (120 grit) before topcoat

Prep standard reference: For any hand-tool cleaning of deteriorated coatings on wood, SSPC-SP3 Power Tool Cleaning applies. For bare wood going to primer, the surface should be clean, dry, and free of mill glaze. See our surface preparation standards guide for the full SSPC/AMPP SP grade definitions.

Stucco

1. Power wash at 1,500–2,000 PSI; do not exceed 2,000 PSI on aged or soft stucco
2. Treat efflorescence with a dilute muriatic acid wash (1:10), rinse thoroughly, allow to dry fully before any coating
3. Crack repair: Hairline cracks (< 1/16") can be filled with a high-quality 100% acrylic caulk or elastomeric filler; cracks > 1/16" require V-grooving, flexible epoxy, and mesh reinforcement before topcoat
4. Assess existing elastomeric layers: Multiple elastomeric coats can trap moisture. If the current coating is bridging cracks and shows excessive thickness, discuss removal options with the owner before adding another layer

Masonry

1. Remove efflorescence by dry brushing, then acid etch if heavy — rinse to neutral pH before coating
2. Repoint mortar joints that are eroded or cracked — painting over deteriorated mortar is a warranty liability; exclude this from scope or price it as a separate line item
3. Seal penetrations at utility chases, windows, and reveals with compatible sealant before applying masonry coating
4. Prime with masonry primer — standard latex primers do not seal alkaline masonry; use a purpose-formulated masonry sealer/primer

Caulking: Sequence and Product Selection

Caulking is one of the most commonly rushed stages of exterior prep. Caulk applied in the wrong sequence or over a dirty substrate fails early and creates warranty callbacks.

The correct sequence: Clean → Repair → Prime bare wood → Caulk → Topcoat

Caulking goes on after the primer, never before. Primer bonds directly to the wood substrate; caulk on unprimed wood loses adhesion at the substrate interface. Topcoating encapsulates the caulk and produces the cleanest finished appearance.

Product selection:

Never use standard silicone caulk on any surface that will be painted — paint will not bond to cured silicone, regardless of what the label claims about paintability.

Coating System Selection

Residential Wood and Fibre Cement

The industry standard is a 2-coat system: one coat of premium 100% acrylic primer (or spot-prime only if existing paint is in excellent condition) + two finish coats of 100% acrylic exterior latex.

Do not use vinyl-acrylic or "vinyl latex" formulations on exterior wood — they are less flexible, have lower adhesion, and are not appropriate for substrates subject to seasonal movement.

Stucco and Masonry — Standard System

100% acrylic masonry paint, two coats. Minimum recommended DFT: 4–6 mils WFT per coat. A quality masonry coating should build adequate film thickness to bridge micro-porosity.

Stucco and Masonry — Elastomeric System

When flexibility and crack bridging are the primary concerns (older stucco with active hairline cracking, EIFS transitions), specify a 100% acrylic elastomeric coating. Elastomerics are specified to ASTM C1585 and bridge cracks up to approximately 1/16" — they are not a structural repair for large cracks.

Elastomeric coverage rate: 50–100 SF/gallon at recommended DFT — significantly lower than standard latex (300–400 SF/gallon). Material costs for elastomeric work are typically 3–4× higher than standard repaint. Price accordingly.

Previously Coated Metal

Spot-prime all rust with a DTM (direct-to-metal) primer that includes a corrosion inhibitor. For structural steel or severely corroded metal, SSPC-SP10 Near-White Blast Cleaning prior to an industrial primer may be warranted — see our professional coating guide for the coating chemistry options.

PCA P10 Production Rates: Exterior Work

Representative exterior production rates (brush/roller unless noted):

Exterior prep labour is almost always 40–60% of total job hours. Contractors who skip this in their estimate are selling prep work at zero margin. Use the bid formula in our estimating guide with prep hours broken out as a separate line item.

Common Exterior Scope Traps

Rot discovery: State explicitly in your contract that if concealed rot is discovered during prep, it will be documented, the owner notified, and remediation priced as a change order. Rot discovered after scraping is never within the original scope unless specifically examined and quoted before signing.

Multiple coating layers: If the existing paint system is more than 3–4 coats thick, delamination between layers is a real risk. A tape test and wet-film build calculation should be in your pre-quote process, not discovered on the job.

Weather windows: Exterior coatings require minimum 40–50°F (product-dependent), dry substrate, and no rain for 24–48 hours after application. Build schedule buffer into multi-week exterior projects. A contract that ties payment milestones to calendar dates is a liability in variable weather — tie milestones to completion of defined phases instead.

Frequently Asked Questions

How many coats does an exterior repaint need? On properly prepared substrate with intact existing paint: typically one coat of spot primer on bare areas plus two topcoats. On bare or heavily stripped surfaces: a full prime coat plus two topcoats. Elastomeric applications are typically one coat at high DFT due to the very low spread rate.

Should I always power wash before an exterior repaint? Yes, for virtually all exterior substrates. The only exception is delicate masonry (sandstone, soft brick) where high pressure can erode the surface. In those cases, low-pressure chemical cleaning and hand scrubbing is appropriate.

What moisture content is acceptable before coating wood? Most paint manufacturers specify ≤ 15%MC for wood substrates. Some specify ≤ 12%MC for dense, resinous species. Use a pin-type moisture meter and test at multiple points, particularly north-facing elevations and ground-line areas which retain moisture longest.

How do I price a job where there is significant rot or caulk failure I can't fully quantify upfront? Include a unit-rate provision in your contract: "Rot remediation at $X per board foot; caulk removal and replacement at $Y per LF, billed as a change order above the first Z linear feet included in base scope." This protects you without requiring a worst-case estimate that loses the bid.