How do you seal PVC trim boards?

PVC (polyvinyl chloride) trim boards are a popular choice for exterior trim due to their durability and low maintenance. However, proper sealing is crucial to prevent water intrusion that can lead to damage. This article will provide a step-by-step guide on how to properly seal PVC trim boards.

Why is sealing PVC trim boards important?

Sealing PVC trim boards is important for several reasons:

• Prevents water intrusion – Unsealed joints and edges allow water to seep in, leading to potential rot, mold growth, and structural damage.
• Maintains appearance – Sealing protects the nice white finish of PVC boards from dirt buildup and UV damage.
• Improves longevity – Keeping water out extends the lifespan of trim boards by preventing cracking, swelling, and delamination.
• Meets building code – Most building codes require exterior trim to be properly sealed as a water barrier.

Neglecting to seal PVC trim boards can void warranties from manufacturers. Sealing should be done upon initial installation and inspected/reapplied as needed.

What supplies are needed?

Sealing PVC trim boards requires just a few simple supplies:

• PVC primer – Softens and prepares the surface for maximum adhesion.
• PVC cement – Permanently bonds trim board joints together.
• Paper towels – For cleaning and smoothing joints.
• Utility knife – Trims away excess cement for a neat finish.
• Wood block – Applies even pressure when joining board ends.
• Painter’s tape – Masks edges for straight sealant lines.
• Silicone caulk – Flexible sealant for gaps and board edges.
• Caulk gun – Dispenses sealant smoothly and evenly.

Be sure to use products designed for PVC rather than standard wood primers, cements, and caulks. Wearing gloves and eye protection is also recommended when working with chemicals.

How to prepare PVC trim boards

Proper preparation is key prior to sealing:

• Trim boards should be cut to size and test fitted before permanent assembly.
• Ensure all joints and ends are smooth, clean, and free of burrs or debris.
• Boards must be dry since moisture will prevent adequate bonding.
• Use PVC primer to roughen and soften joint surfaces for better adhesion.
• Only prime as much area as can be sealed in a few minutes before primer dries.

Rushed preparation leads to failed seals and leaks down the road. It also helps to do any final sanding or finishing prior to sealing.

How to join and seal PVC trim board lengths

When joining PVC boards end-to-end, follow these steps:

1. Run a 1/8″ bead of PVC cement along the end of one board.
2. Immediately press the end of the second board firmly against the first.
3. Use a wood block to apply even clamping pressure and maintain proper alignment as the cement sets.
4. Hold joined boards together for at least 30 seconds to allow initial bond.
5. After 10-15 minutes, scrape away any excess cement squeezed out of the joint with a utility knife for a smooth finish.
6. Allow several hours for cement to fully cure before handling boards.

The cement forms a permanent, watertight fusion between the ends of the trim boards. Take care not to get any cement on outward facing surfaces meant to be painted or finished.

How to seal inside corners

Non-mitered inside corners where boards meet at a 90 degree angle should also be sealed:

1. Apply PVC primer to the inside edges of the adjoining boards.
2. Run a 1/8” bead of PVC cement along the edge of one board.
3. Immediately press the primed edge of the second board against the cement to bond.
4. Use a wood block or rubber mallet to apply firm, even pressure at the joint for maximum surface contact and adhesion.
5. Hold in place for 30-60 seconds.
6. After 10-15 minutes, scrape away any excess cement for a clean interior corner.
7. Allow cement to fully cure before continuing installation.

Proper inside corner sealing prevents moisture from collecting in hidden cracks. Cementing provides a continuous water barrier.

How to seal outside corners

For attractive finished looks, outside corners are often mitered at a 45 degree angle rather than butted at 90 degrees. Sealing steps include:

1. Cut and test fit the mitered outside corner to ensure proper alignment.
2. Use PVC primer on the mitered edges to enhance bonding.
3. Apply a 1/8” bead of PVC cement along one mitered edge.
4. Immediately fit the corner together and press firmly for maximum surface contact.
5. Use clamps or tape to hold the corner tightly together as the cement cures.
6. After 10-15 minutes, remove any excess cement squeezed from the joint with a utility knife.
7. Allow cement to fully cure before disturbing the sealed corner.

Mitered outside corners have more bonding area and look more attractive when installed properly with trim boards meeting in a tight, hairline joint.

How to seal around windows, doors, and other openings

Gaps around windows, doors, and other exterior openings allow air and water penetration. Sealing these potential entry points is critical.

1. Cut PVC trim boards to tightly fit openings, allowing space for caulk sealant.
2. Seal all joints between trim boards with PVC cement as previously described.
3. Mask trim board edges with painter’s tape for straight caulk lines.
4. Run a steady 1/4” bead of silicone caulk around all exterior edges.
5. Use a caulk tool, wet finger, or wooden stick to smooth the caulk into a neat fillet.
6. Remove masking tape immediately after tooling caulk.
7. Allow caulk to fully cure for 24-48 hours before painting or exposure to weather.

Top quality silicone or urethane caulking offers superior flexibility and adhesion to seal gaps while allowing for normal expansion and contraction. Periodic caulk inspection and touch-up maintenance may be needed.

How to seal the bottom edge of trim boards

Preventing water wicking up into the bottom edges of trim boards is especially important. Effective techniques include:

• Allowing a 1/4″ gap between the bottom trim edge and horizontal surfaces for drainage and caulking.
• Applying caulk fillet bead along the underside bottom edge and smoothing into any gaps.
• Installing trim board bottom edges on Z-flashing for a drip edge.
• Using J-channel along the bottom to capture trim ends and direct water away.

Keeping the bottom trim edge well sealed and not in direct contact with horizontal surfaces avoids Potentially costly damage.

Finishing sealed trim boards

Once all PVC trim board joints have been cemented and edges caulked, appearance and weather resistance can be enhanced by:

• Filling any remaining cracks or defects with silicone caulk.
• Sanding surfaces lightly to improve caulk and cement adhesion.
• Priming and painting with 100% acrylic latex paint formulated for plastics.
• Applying clear PVC weatherproofing sealant for UV protection.

Follow all manufacturer’s directions for finishing products to achieve the best results on sealed trim.

Conclusion

Proper sealing is essential for both the appearance and performance of PVC trim boards on a home’s exterior. By following the sealing steps outlined above, moisture intrusion through joints, gaps, and edges can be minimized for lower maintenance and longer lasting trim board service life. Allowing adequate cure times for cements and caulks to fully bond ensures the integrity of the seals against weather and normal expansion/contraction forces. With just a little time invested upfront in careful sealing, PVC trim boards will retain their like-new appearance for years before requiring any touch up or resealing.