Surface Preparation by Substrate
The general sequence in the previous chapter changes in the specifics for every material you meet, and the exam tests those specifics substrate by substrate. This chapter is organized the same way, as a working reference: wood, engineered and cement-based siding, plastics and metals, masonry and concrete, and previously finished interior surfaces. It is the second of three chapters on surface preparation, which is about 30 percent of the official CSLB C-33 outline.
Wood, Bare and Weathered
Bare wood is sanded in the direction of the grain, because cross-grain scratches show through a finish and are nearly impossible to remove once a coating is on. Knots and sap streaks are sealed with a stain-blocking or shellac primer before the primer coat goes over the field. Weathered wood needs more than a wash. Bare fir siding exposed for eight months on a north wall develops a gray, fuzzy layer of ultraviolet-degraded fibers, and priming over that layer bonds the coating to loose fiber; the layer must be sanded off to sound wood first. Smooth-surfaced redwood decking has a slick planer-produced sheen called mill glaze, and a penetrating stain cannot penetrate through it, so the glaze is broken by sanding. Wood species matter for the primer choice: redwood and cedar require a stain-blocking primer rated for tannin bleed, and shellac primer is the wrong choice on an exterior fascia because shellac is brittle and water-sensitive and is not intended for exterior exposure. Dry, weathered but sound siding calls for an oil or alkyd penetrating exterior wood primer, which soaks into the loosened surface rather than sitting on it. Ornate exterior millwork that has to be stripped without losing its profiles is handled with a low-pressure soft media system or a chemical stripper suited to the millwork, never with aggressive blasting.
Engineered Siding, Fiber Cement, Vinyl, and PVC Trim
Modern exteriors are rarely all solid wood, and each manufactured product has one preparation detail that decides whether the job holds. Factory-primed fiber-cement siding arrives coated on the faces and edges as delivered, but every field cut exposes raw board, so the cut end of each board needs a coat of primer before it is coated out. Vinyl siding carries a color constraint rather than a preparation constraint: dark colors absorb far more solar heat, and vinyl will warp if the surface temperature climbs beyond what the material tolerates, so a homeowner who wants tan vinyl repainted deep charcoal must be steered to a vinyl-safe color formulated to reflect infrared. Cellular PVC trim comes with a slick factory skin and mold-release residue from the extrusion process; the release residue is solvent-wiped off and the surface scuffed dull, because a coating cannot bond to a release agent whose entire purpose is preventing adhesion. Anodized aluminum window frames that show a dull white powder are oxidizing, and that oxidation is washed off before a primer rated for aluminum is applied. Galvanized steel is the substrate most often ruined by rushing. New galvanized flashing and gutters carry manufacturing oils, and a chalky white bloom on stock delivered in a wet stack is wet storage stain, which must be removed. In both cases the zinc surface is cleaned of oils and stain first, then primed with a primer specifically rated for galvanized metal or coated with a galvanized-rated direct-to-metal acrylic. A downspout that peels in sheets after one summer, despite a correct primer, almost always had oils left on the zinc.
Ferrous Metal and the SSPC Surface Preparation Standards
Bare iron and steel are primed with a rust-inhibitive metal primer, and the degree of cleaning required is not left to judgment on commercial work; it is specified by a standard. The SSPC surface preparation standards, now published under AMPP, define levels of cleanliness that a specification can call out and an inspector can verify. SP1 is solvent cleaning, the removal of oil, grease, and soluble contaminants, and it comes first in every sequence, because abrasive blasting over grease drives the contaminant into the profile. That is why a specification requiring SP6 commercial blast cleaning on structural steel that arrived coated in grease begins with SP1 solvent cleaning, not with the blast. SP2 is hand tool cleaning and SP3 is power tool cleaning, both of which remove loose rust, loose mill scale, and loose paint down to a firm surface while leaving tight adherent material in place; they are the appropriate standards for an exterior steel stair with scattered rust blisters and tight mill scale when the budget rules out blasting. SP6 is commercial blast cleaning, a specified degree of blast that removes nearly all visible contamination. Blasting also creates an anchor profile, the roughness of the cleaned steel, and profile depth is inspected because the roughened surface gives the primer mechanical grip; too shallow a profile and the coating has nothing to hold. A wrought iron gate with flaking rust scale and pitting must have the scale removed mechanically to tight metal before a rust-inhibitive primer can do anything. Freshly blasted steel is also perishable: leaving it overnight allows flash rust to form, which compromises primer adhesion, so blast and prime within the specification's stated window.
Concrete, Masonry, and Stucco
Cementitious surfaces present three obstacles at once: alkalinity, moisture, and a surface that is either too dense or too porous. A hard-troweled warehouse slab is far too smooth and closed for a high-build epoxy to grip, even when it is clean, so the epoxy requires mechanical abrasion, usually diamond grinding or shot blasting, to open the surface. Acid etching is the lower-cost alternative, and the step that decides whether the coating holds is not the etch itself but the rinsing and vacuuming afterward until the slab is free of residue; acid salts left on the slab are a bond breaker. Fresh masonry must cure before most coatings go on it, because it is alkaline and attacks the film, and a stucco wall floated eight days ago should either cure longer or receive an alkali-resistant primer. Sandblasting stucco to strip failing paint quickly is a false economy, because blasting erodes the stucco surface itself and leaves an unsound face that no coating will hold. Hairline cracks in stucco are cleaned and filled while dry before an elastomeric coating goes over them, and ordinary wall paint will not bridge a moving crack no matter how many coats are applied, because a conventional paint film is too thin. Concrete masonry unit walls need a block filler, a high-build product designed to fill the pores and voids of the block and create a smooth base, and its coverage is nothing like a topcoat's, a point that catches estimators out. Dust generated by dry cutting or grinding concrete during preparation is respirable crystalline silica and is controlled under Cal/OSHA.
Previously Finished Interior Surfaces
Recoating something that already has a hard, cured finish is an adhesion problem, and the answer is always some combination of degreasing, mechanical abrasion, and a bonding primer. Hard, glossy oil enamel on 1960s door casings that the owner wants finished in waterborne enamel gets degreased, sanded until the gloss is dull, and then coated with a bonding primer, because a waterborne film has no chemical affinity for a fully cured alkyd. Kitchen cabinet doors with a factory conversion varnish are the hardest case: degrease, scuff-sand every face including the profiles, then use an adhesion primer, so the new system has both a mechanical key and a chemical grip. Melamine laminate can honestly be coated after degreasing, scuffing, and priming, and the professional answer is to say so rather than either refusing the work or promising a factory finish. MDF has a specific quirk: the cut edges are open fiber and will absorb coating indefinitely until they are sealed and filled, which is why they stay rough and dull no matter how many topcoats go on. Popcorn acoustic texture on a 1968 ceiling must be sampled and analyzed for asbestos before a scraper touches it, and drywall paper facing torn during wallpaper removal is sealed with a solvent-borne or shellac sealer, then skimmed, sanded, and primed, because skimming directly over torn, wet-swollen paper produces bubbles. Any glossy or previously enameled surface is de-glossed before recoating for the same reason throughout: a dull, abraded surface bonds and a polished one does not.
Last updated: September 2026