Bathroom Vanity Doors in a Coastal Guesthouse
A workshop has an order for 12 bathroom vanity door panels, each 600 mm × 400 mm × 18 mm, for a guesthouse 800 m from the sea. The store holds 18 mm standard MDF, 18 mm moisture-resistant (MR) MDF, 18 mm exterior-grade plywood and 18 mm solid meranti. Adhesives on the shelf are PVA D3, polyurethane (PU) and solvent-based contact adhesive. Finishes available are nitrocellulose lacquer, water-based acrylic lacquer and two-pack polyurethane. The client wants a painted white finish and can fund one upgrade only: either MR MDF instead of standard MDF, or two-pack polyurethane instead of nitrocellulose, but not both. The doors will be hung with concealed hinges and will see about 30 minutes of steam each day; the bathroom extractor is inadequate. Workshop humidity is 75% RH. The joiner proposes standard MDF with nitrocellulose lacquer because it is cheapest and sprays easily.
1. Evaluate the joiner's proposal. Recommend a board, adhesive and finish, and justify each choice against the service conditions.
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The proposal is not fit for the service conditions.
- Standard MDF is not moisture-resistant. Its cut edges and hinge screw holes absorb water vapour, swell and lose screw-holding strength; at 75% RH and daily steam this will happen quickly.
- Nitrocellulose lacquer is moisture-permeable and softens under warm, humid conditions; white paint will show blistering and cracking.
- PVA D3 is an interior adhesive and will creep or fail under damp conditions. Contact adhesive is unsuitable for structural joints because it creeps under sustained load; it is also solvent-heavy in a small workshop.
Minimum specification: use MR MDF for the board upgrade, PU adhesive for joints and edge sealing, and a water-resistant finish. If only one upgrade is funded, spend it on the board, not on nitrocellulose; specify a water-based acrylic system with all edges sealed, or explain that the budget does not buy a durable door. The best solution is MR MDF or exterior plywood plus two-pack polyurethane. Exterior plywood is stronger at the edges but needs edge sealing and filling before painting. Solid meranti is not a direct substitute: a 600 mm wide slab will move, so it would need a framed construction.
Detailing: seal all cut edges and hinge holes with PU or edge banding, prime bare edges, and use concealed hinges with machine screws into sound material, not just the MDF face.
2. The client asks why solid meranti at the same thickness cannot simply replace the board. Explain the movement and finishing issues, and state what would be needed if meranti were used.
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Solid meranti moves across the grain as humidity changes; a 600 mm wide panel could move 2-4 mm seasonally. If used as a slab, the paint film will crack at the joints and the door may cup. A proper meranti door would be frame-and-panel: narrow stiles and rails with a floating panel, allowing 2-3 mm clearance in the grooves, and all faces sealed equally. The panel should not be glued along its full width. The finish must be flexible and water-resistant; two-pack polyurethane is better than nitrocellulose. The hinge positions must be in the stiles, not in the panel. This is more work than the MDF option and would remove the cost advantage.