Understanding the substrate and proper preparation techniques is crucial when painting dimension lumber such as columns, beams, joists, decking, siding, and fencing. Correctly analyzing the degree of surface degradation and surface preparation requirements will determine whether your repainting job lasts, or whether you’ve wasted a lot of time and money.

Substrate Types and Treatments
Exterior siding and fencing commonly use softwoods like cedar, pine, fir, or spruce. These woods come in smooth or rough finishes, impacting the type of paint and finish used. Smooth surfaces can accommodate any gloss level, while rough surfaces typically use flat to semi-gloss finishes to mask texture inconsistencies.

There are a few special considerations to be aware of. Cedar and similar woods contain tannins that can bleed through latex coatings and cause stains. To prevent this, use alkyd or oil-based primers. Pine and fir often have sap, which can lift paint films if improperly sealed. You’ll need to apply knot sealers to problematic areas to mitigate this issue.

Surface Preparation
Your role in effective surface preparation is not just important, it’s essential for long-lasting finishes. Environmental factors, including moisture and temperature fluctuations, can lead to paint failures. This makes proper assessment and preparation critical.

Know your degrees of surface degradation. The degree of degradation will determine how you prep and apply coatings.

1. DSD-0: Sound Wood 
   • Ideal for color or sheen changes without damage.
   • Requires minimal preparation, mainly cleaning and light sanding for adhesion.

2. DSD-1: Slightly Deteriorated 
   • Requires sanding of sags, runs, and uneven areas.
   • Remove dust, dirt, chalk, and ensure dryness.
   • Clean oil and grease, dull glossy finishes, and treat mildew.
   • Spot prime bare patches before further application.

3. DSD-2: Moderately Deteriorated
   • Similar to DSD-1 but with more thorough cleaning and sealing of sap, pitch, and stains.
   • Remove loose paint, fill cracks, and prime exposed areas.
   • May require complete paint removal if severe blistering or delamination is present.

4. DSD-3: Severely Deteriorated
   • Extensive preparation involving sanding, cleaning, and sealing.
   • Remove loose knots, fill cracks, and apply stain-blocking primers.
   • Requires priming all areas as specified.

5. DSD-4: Substrate Damage
   • When wood is beyond repair, replacement is necessary.
   • Address dry rot, insect damage, deep grain cracks, and severe dimensional changes.

Primer Recommendations
For bare exterior wood surfaces, MPI #5 primers are recommended for their quick drying time and better penetration. For repainting projects, Latex primers (MPI #6) offer improved resistance to blistering but may not penetrate as well. Use oil or alkyd-based primers for woods that are prone to extractive bleeding.

By following these guidelines, repainting projects can be durable and aesthetically pleasing on dimension lumber. A proper finish ensures the longevity and attractiveness of commercial assets.