1. Rake out the old seams
Work the reefing hook along each seam until you reach clean timber on both faces. Stop before you hit the fastenings and take care not to widen the seam edges.
2. Prepare and prime
Vacuum out the dust, let the deck dry thoroughly, then prime both seam faces. Unprimed timber sucks the oils out of the compound and the seam fails within a season.
3. Drive the cotton
Loop the cotton into the seam and set it with the iron in even strokes so it sits just below the surface. Consistent tension matters more than force — over-driving can spring the plank.
4. Pay the seams
Fill the remaining depth with marine seam compound, slightly proud, and tool it flat. Leave it to cure fully before sanding flush with a hard block so the seams stay level with the deck.
Aftercare
Keep the deck wet in dry spells so the planks stay swollen, and inspect seams each spring. Spot-recaulking a metre of seam is a morning's work; leaving it is a plank replacement.