Finishing edges
A major production advantage of MDF compared with other wood based sheet materials is the relative ease of finishing panel edges without the need for time consuming filling treatments or the application of lippings or other adhesive bonded edging materials. This characteristic derives from the substantially uniform density gradient across the thickness of MDF and the absence of core voids.
MDF panel edges should be cleanly cut with sharp tools and be free from saw or cutter marks. The high cutting speeds and controlled feed rates associated with CNC routers are particularly advantageous producing smooth edges requiring a minimum preparation before finishing.
Sanding
Panel edges can be sanded with any one of a number of different profile sanders chosen with regard to the shape of the profile and the scale of production. Apart from hand sanding with flexible blocks, possible systems include sanding stars, woodworking grinding wheels, flexible belt sanders with moulded pad backing or profiled sanding heads. As a general rule 80/100 grit should he used for the removal of cutter marks, and 120/150 grit as a final treatment before finishing.
More companies are changing over to woodworking grinding wheels, partly because of their ability to maintain the detail of complicated mouldings during long runs but also because the frictional heat generated at the point of contact is thought to have a burnishing effect on the MDF edge reducing the uptake of sealer.
Sealing
Absorption of lacquer or paint into the machined edges of MDF can be prevented by the application of an appropriate sealer. Several different resin types including shellac, sanding sealer and diluted polyvinyl acetate (PVAC) adhesive have been used for this purpose with varying degrees of success. Nowadays, the best results are obtained with specially formulated, high solids sealers based on two component catalysed resins, polyurethane, polyester and acid catalysed systems in particular. These materials effectively seal any porosity in the edges of the MDF and, advantageously, the cured film is resistant to softening by solvents in subsequently applied finishing materials
The following procedure is recommended when using two component, catalysed sealers:
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Apply sealer to board edges by wiping, brush, roller or spray.
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Allow the sealer to cure for the suppliers specified period at room temperature. A shorter period at elevated temperature may be acceptable subject to a minimum flash off time before applying heat.
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Denib by hand using 360 grit paper, a flexible pad, or abrasive brush fitted to an air operated grinder.
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On automated lines, the sealed edges can be denibbed automatically using an abrasive brush or sanding disc.
Finishing
The sealed edge can be stained, if required, and then finished with one or two coats of clear or tinted lacquer to match the finish on the surface. Alternatively, the surfaces and sealed edges can be finished with one or two coats of paint. Assurance of compatibility of sealer, stain and lacquer should be obtained from the suppliers.
