Finishing sheet goods (plywood, MDF, MDO)

Overview

Plywood, MDF and overlaid panels finish differently from solid wood in three ways the sources opened agree on: the face veneer is thin, so heavy sanding goes through it; the face is porous and needs sealing for an even coat; and the edges are the weak point. This entry covers plywood most fully, because the sources do. They say little or nothing about MDF edge sealing in furniture, melamine and edge-banded panels or already-prefinished plywood, and this guide does not invent it.

When to use

• Cabinet interiors and shop cabinets in plain or veneered plywood, where speed and a durable, smooth surface matter more than furniture-grade finish (Southwick, American Woodworker).
• Exterior-grade panels, where the Wood Handbook has specific advice on edges and overlays.
• Not for solid wood (Sanding schedule) or for face veneer you intend to stain, where see Finishing veneers.

Materials and tools

Dewaxed shellac (Zinsser SealCoat, a 2-lb cut, is Southwick's pick), a roller with a 1/4-inch mohair nap or an HVLP gun (Spraying with HVLP), 100 and 320 grit paper, microfibre cloth (Dust removal and tack cloths), and a waterborne topcoat (Waterborne acrylic and urethane-acrylic topcoats or Water-based polyurethane).

Steps

1. Sand lightly. Southwick sands plywood with 100 grit on a random-orbit sander to remove scuffs and minor scratches, in about ten minutes. Flexner starts pre-sanded veneered plywood and MDF at #120 to #150 because #80 risks sanding through the veneer.
2. Remove the dust with a tack cloth or vacuum.
3. Seal with dewaxed shellac. Roll or spray one coat, starting in the middle and working toward the edges (Dewaxed shellac). Southwick says it seals porous plywood, dries fast and is compatible with virtually all other clear topcoats.
4. Dry 15 to 20 minutes, then lightly sand off raised grain and nibs by hand with a block and 320 grit. Clean with a dampened microfibre cloth.
5. Topcoat. Southwick applies one thick coat of waterborne floor finish with a floor applicator, or sprays it with a half-lapping pattern, and it is dry to the touch and ready to move in 20 to 30 minutes.
6. Seal the edges. The Wood Handbook says the edges of plywood, OSB and fiberboard are especially vulnerable to water because they contain end grain, and recommends sealing them with a water-repellent preservative, thinned oil-alkyd primer or an edge sealer made for the purpose (Finishing end grain).
7. When spraying, clean all shellac out of the gun with denatured alcohol before switching to waterborne; Southwick says any residue is "guaranteed to gum up the works".

For stained plywood, treat the face like any veneer: a stain controller is often needed (Pre-stain conditioner and washcoat, Gel stain).

Common problems

• Sanded through the face veneer. Start finer and use a light touch. For a repair see Finishing veneers.
• Rough surface after the first coat. Raised grain; sand lightly as in step 4 (Raising the grain).
• Lathe checks in smooth plywood. The Wood Handbook describes fine cracks in the veneer face that open with wetting and drying cycles and affect paint durability on exterior plywood; they are less of a concern for interior cabinets.
• Penetrating finish on MDO. The Wood Handbook says paper overlays hold film-forming finishes better but do not accept penetrating finishes. OSB and fiberboard without an overlay are not generally recommended for exterior exposure at all.
• Edges swell and finish peels. Unsealed edges; see step 6 (Outdoor finishing).

Safety and environment

Spraying or rolling shellac over a large sheet saturates the air with solvent fumes; Southwick says to wear a respirator and ventilate. Plywood and MDF dust is wood dust (and MDF dust contains the resin binder): use extraction and a respirator. Shellac is dissolved in denatured alcohol, which is flammable.

Related

• Dewaxed shellac, Water-based polyurethane, Waterborne acrylic and urethane-acrylic topcoats
• Finishing veneers, Finishing end grain, Sanding schedule
• Spraying with HVLP, Outdoor finishing