Surface preparation may be the single most important factor to a successful result. The quality of the decorative finish will be impacted by the quality of the surface. An improperly or poorly prepared surface may cause an uneven appearance, uneven adhesion or a generally unprofessional look. Bumpy (raised) or pitted (recessed) areas on the surface can detract from an otherwise attractive finish. Blemishes, flaws, artifacts and other irregularities may not only be noticeable through the decorative finish, but may be accentuated/magnified. Greasy or dusty surfaces will not bond and must be thoroughly cleaned prior to application. However, purposefully textured surfaces are the exception. For the most part, a beautiful finish surface is to be admired for its attention to detail and care. The extra time and effort expended in surface preparation pays.
In addition to preparation for the express purpose of correcting blemishes and basecoating/sealing, the project may require decorative surface preparation; i.e. creating a basecoat, pattern or subfinish that will support the objective finish. Not only will different effects require or be augmented by specific surface details, but the choice of surface details may have artistic or even project efficiency impact on the results, e.g.
- A purposeful basecoat color may be required for a skip-troweled effect.
- An appropriate basecoat color may facilitate better, easier and faster color coverage by hiding an existing contrasting color.
Not all surfaces require the same degree of preparation. The need and method is determined by a combination of the existing condition, existing composition and materials used (if known or unknown) and the target decorative finish. One thing is for sure though – shortcuts or avoided procedures may result in something less than desirable and regrets. Don’t skimp on time and effort here. The tradeoff is not worth it.
A clean, undamaged surface will likely take a new painted finish without adhesion problems. However, it may be unclear as to the history of the existing finish. For example:
- Surface grease, hidden to the naked eye, will impede adhesion or cause later failure.
- A previously wallpapered wall, cleaned but with invisible traces of wallpaper adhesive, will likely yield bubbles in the new finish when waterborne materials reactivate traces of adhesive.
- Glossy surfaces may need physical (sanding) or chemical deglossing to provide “tooth” (grip) for the new finish being applied.
- Bumps, dips, dried bits and old paint drips will present through the new finish. A new coat of paint will not suffice to cover/hide blemishes.
Existing Composition and Materials Used
- When applying waterborne materials (e.g. Latex, acrylic) over an existing waterborne material surface, priming is not required.
- When applying waterborne materials over an existing oil-based material (alkyd), priming is required. Recommend using an oil-based primer.
- When applying oil-based materials over an existing oil-based or waterborne material, priming is not required.
- New Sheetrock™ requires priming. Recommend using a waterborne primer.
- If the surface has been spot patched (spackled), priming is required. Primer type is determined by surface type (oil or water).
- When in doubt, prime.
- When in doubt as to type of primer, it cannot hurt to use oil-based.
- An extra coat of paint does not equate to the use of a primer layer. Paint and primer have different chemical and physical characteristics.
Target Decorative Finish
- Adhesion and material compatibility issues aside, the need to spend time on surface preparation and the degree to which it is required has much to do with the type of finish planned.
- In the event that a textured surface is being applied, the amount and degree to which surface preparation is required may be less, however this is dependent on the circumstances. Textured finishes hide a multitude of sins. A new texture layer may camouflage an existing finish’s problems. Severe surface problems such as holes, cracks, loose material and other major flaws need to be addressed however, as even a rough texture may not disguise issues underneath.
- If the finish calls for texture, there is some leeway for imperfect walls, however heavy imperfections will show through.
- Occasionally, the choice of new finish is dictated by the existing finish conditions and the appetite and budget for preparation work.
- Fine, smooth finishes may draw attention to or amplify blemishes underneath. Do not apply over textured walls. Skim coat first. Burnishing will bring out every imperfection in the wall and will show up as shiny dark spots.
Ways and Means of Surface Preparation
A. Patching and spackling
- Spackle and joint compound are not the same, though frequently used interchangeably for small repairs. Spackle contains oil-based materials and will not shrink when dry. Joint compound is basically powdered clay mixed with water. It may crack if applied too thick.
- Wall cracks may reappear after patching and painting. Prevent this by applying fiberglass mesh tape over the crack but beneath the Spackle.
- Start with coarser grit and move toward finer (higher grit number) sandpaper.
- Wet sanding will create a very smooth surface. Be sure to clean off residue before painting.
C. Hole filling
- Nail holes in newly affixed molding and those left over from removed picture hangers need to be filled.
D. Priming and sealing
- New porous construction materials require sealing prior to the application of the finish, including raw wood, Sheetrock, plaster and concrete.
- After drying, lightly sand primed area to remove raised dried bits of material left by the roller.
- Dark marks, scuffs and ink stains are likely to show through paint, even if multiple layers are applied. Prior to painting, apply a stain-killing primer to block any bleed through, especially due to materials used in ink.
E. Caulking and wood filler
- Used to fill gaps, cracks and small holes.
F. Surface irregularities are frequently difficult to see or detect.
A commonly used technique for making this easier is to shine a bright light (spot or floodlight, handheld) at an angle to the surface. Raised or depressed areas will show up as shadows.
G. When staining new wood, apply wood conditioner first.
This stabilizes and evens the absorption rate of the wood surface, helping to prevent splotches (dark/light areas).
H. When painting new wood, apply a stain-killing primer to knots to prevent discoloration of the painted finish due to sap seepage.