Lacquer Finishes

The smooth and stable surfaces of MDF and the well compacted machined edges are prerequisites for successful finishing with a wide range of liquid coatings. Pigmented lacquers are widely used to create strong single colour effects or, using modified lacquers and a good finishing technique, unique pearl, metallic, marble or other dramatic effects. Alternatively, the natural appearance of the MDF surface can be enhanced using a transparent stain with a clear lacquer topcoat.

The selection of the finishing system on the basis of chemical type will be dependent upon the scale of production, application equipment, drying facilities and the expected performance of the finish in relation to the conditions of use.

Surface preparation

The surfaces to be finished should be free from dust or sanding marks. MDF, normally supplied with a 100/120 grit or better surface, will be suitable for most matt finishing treatments without further sanding. An additional light sanding with 150/180 grit is recommended when using high gloss finishes or where a minimum coating thickness is required.

Edge preparation

As the edges of MDF are more absorbent than the surfaces, finishes applied to the edges may differ in appearance from the surface finish. In particular, the increased absorption of stain into the edges will result in a darker colour compared with the surface. With lacquers, a higher coating weight may be required on the edges to accommodate this increased absorption.

In order to control the problem, lacquer manufacturers have developed specially formulated sealers, usually two pack, at a high solids content for application to the MDF edges before finishing in the normal manner.

Pigmented lacquer

Intense white or strong colour finishes can be achieved on MDF by applying two or more coats of pigmented lacquer. The method of applying the lacquer will be dependent upon the nature of the product, in the form of individual components or built-up units, and the number of items.

Small companies without sophisticated finishing equipment can achieve good results using DIY paints applied by brush with a light denib between coats. Better results can be achieved in a shorter time using commercial lacquers based on nitrocellulose, acid catalysed, polyurethane or polyester resins applied by hand spray.

A typical finishing sequence would start with a sealer applied at a low coating weight. The sealer may be clear or high solids content, pigmented to mask the MDF surface and reduce the number of finishing coats. Alternatively, a superior finish can be obtained by using a high solids pigmented primer instead of or in addition to a clear sealer. After denibbing with 320 grit paper, one or more coats of pigmented lacquer should be applied to complete the finish, taking care to ensure compatibility between the sealer and lacquer.

Companies finishing large numbers of items of similar shape, kitchen or bedroom unit doors for instance, would use integrated finishing lines incorporating automatic spray equipment and forced drying ovens.

Note should be taken of any national regulations relating to the safe handling and application of solvent based finishes, including any limits on the emission of volatile organic compounds including solvents. In this context, successful results can be achieved using water borne finishes.

Stain and clear lacquer

As an alternative to obliterating the MDF with pigmented lacquer, the appearance of the finished product can be enhanced by taking advantage of the variable absorption of a stain applied directly to the surface to highlight the fibre structure.

Solvent borne stains applied directly to the MDF surface will wet the surface effectively and ensure an even colour. Water borne stains can also be used but the waxes sometimes added to fibreboards to improve their water resistance may result in uneven absorption of the stain and a consequent variation of colour.

The stained surface can be protected by one or two coats of clear lacquer with a light denib with 320 grit paper between coats. Deeper colour effects without obliterating the character of the MDF surfaces can be achieved by lightly tinting the lacquer used for the top coat.

High gloss finishes

The high stability of MDF surfaces and edges can be used to good effect for the production of gloss finishes using a high build coating based on polyester resin. At one time, a polyester finish applied by spraying would need to be mechanically burnished to achieve a high gloss surface free from imperfections. Using suitably formulated lacquers, high gloss finishes can now be obtained directly from the spray gun without a subsequent burnishing treatment. As a further development some companies are now applying a polyester base coat to develop the colour on MDF with a clear lacquer top coat to protect the surface and enhance the gloss effect. Non yellowing clear lacquers must be used on white and light colour base coats to maintain the colour.

Flat panel finishing lines

For large scale production of flat panels, a finishing line consisting of a forward roller coater to apply a sealer followed by one or more curtain coaters to apply the finishing coats and a drying oven using hot air, possibly with an infra red heating zone would be most appropriate. Panel edges would be sealed and finished off the line by spraying the exposed edges of stacked panels. Acid catalysed or polyurethane finishes would normally be used for this process.

Some of the environmental problems resulting from the use of solvent based coatings can be eliminated by using UV curing systems based on polyester or acrylic resins. Although the full cure of some of the early pigmented polyester finishes was inhibited by absorption of the radiation by the pigment particles, these problems have been overcome by developments in resin and pigment technology and in the lamps themselves.

The UV curing process at its present stage of development is limited to clear lacquers and relatively low coating weight pigmented lacquers.

The possibilities for a combined finishing system incorporating UV cured sealers and solvent based top coats, either polyurethane or acid catalysed are illustrated in the following examples.

a. High gloss finish

- Roller coated UV sealer, about 30 g/m2
- Gel cure using 80-100 w/cm UV lamp.
- Reverse roller coated UV primer, about 90 g/m2
- Curing with two 80-100 w/cm UV lamps
- De-nibbing with 240 grit paper
- Reverse roller coated UV primer, about 90 g/m2
- Curing with two 80-100 w/cm UV lamps
- De-nibbing with 320/360 grit paper
- One coat of high gloss pigmented polyurethane or acid catalysed lacquer, about 130 g/m2
- Sanding with 1000 grit paper and polish

b. Matt finish

- Roller coated UV sealer, about 30 g/m2
- Gel cure , using 80-100 w/cm UV lamp.
- Reverse roller coated UV primer, about 90 g/m2
- Curing with two 80-100 w/cm UV lamps
- De-nibbing with 320/360 paper
- One coat of matt poly-urethane or acid catalysed lacquer, about 130 g/m2

c. Pretreatment of edges

- One coat of polyester or other two pack primer or two coats of this primer applied within 10/15 min to achieve a total build of 240-250 g/m2
- Dry for 18-24 hours at room temperature
- De-nibbing with 320/360 grit paper

Finish performance

The performance of any finish selected for use on MDF in relation to the requirements of the application will depend upon the nature of the resins and the effects of any additives used in the formulation.

Finish performance is assessed using established national or international standard test methods to assess the resistance of the finished surfaces to wet and dry heat, spilled liquids and different types of mechanical damage including, indentation, scratching and general wear due to abrasion or scuffing.

Satisfactory performance of domestic furniture can normally be achieved by the use of good quality precatalysed lacquers based on nitrocellulose resins. Contract furniture and interior fitments subject to heavy duty use will normally be finished with two component acid curing, polyurethane or polyester systems.

Where flame retardant MDF is used for applications subject to fire control regulations, the finishing system will normally have to satisfy specified surface spread of flame criteria.

Exterior MDF used outside should be finished on all surfaces and edges with approved exterior durability finishes.