Sealing Joints Between Plywood Sheets for In-Situ Architectural Concrete Finishes

Introduction

Producing in-situ architectural concrete with special finishes requires precise treatment of formwork joints. Even when using factory-prepared plywood, poor joint sealing can result in grout loss, surface staining, and visible imperfections. This guide explains how to correctly seal joints between plywood sheets to achieve a flawless architectural finish.

Factory Cut Film-Faced Plywood and Formwork Construction

We are using factory cut film-faced plywood, specially manufactured with two sides coated in a durable, waterproof film. Full-size plywood sheets will be used wherever possible, and site cutting will be minimized. All shutters are constructed with double layers of staggered plywood sheets, screwed from the back face to avoid visible fixing points.

When cutting plywood on site, operatives will use a circular saw with a fine-toothed blade and a straight edge guide. All exposed edges will be treated with a sealing varnish to prevent moisture absorption.

Corner joints are cut at 45 degrees and require careful varnishing to prevent localised darkening due to increased water-to-cement ratios. Grout loss at these joints can also lead to honeycombing. Horizontal joints are particularly sensitive because trapped mould release oil can stain the concrete, unlike vertical joints where gravity helps drainage.

Why Sealing Plywood Joints Matters

– Prevent grout and water loss.
– Achieve uniform surface appearance.
– Minimise formwork edge damage.
– Prevent staining from trapped mould release oil.

How to Seal Joints Between Plywood Sheets

1. Sheet Preparation:
   – Cut edges cleanly using a straight guide.
   – Apply a high-quality varnish such as Induline SW-910 to all cut edges
2. Butt Jointing:
   – Butt sheets tightly together with minimal gaps.
   – Apply a decorator’s caulk like Soudal Decorators Caulk between sheets. This product does not react with wet concrete and physically fills small gaps better than varnish alone. Wipe away excess caulk immediately.Important: Avoid using standard silicone sealants, as they can react chemically with concrete and cause permanent staining.

Construction Joints

Apply decorator’s caulk to the edge of one plywood sheet before butting the next sheet. Remove any excess material for a clean, uniform joint.

Practical Tips

– Inspect joints before each concrete pour.
– Seal all cut edges and 45-degree corner cuts.
– Avoid site cuts where possible to minimise joints.

Common Mistakes to Avoid

– Not sealing edges properly.
– Using reactive silicone that stains the concrete.
– Leaving gaps unfilled.
– Failing to wipe excess caulk before pouring.

Conclusion

Sealing plywood joints correctly is essential for achieving premium in-situ architectural concrete finishes. Combining careful cutting, edge sealing with varnish, and gap filling with non-reactive decorator’s caulk ensures defect-free, professional concrete surfaces. This disciplined approach is critical to delivering the high-end finishes demanded by modern construction projects.