Overcoming Edge Cracking Problems in coated MDF Board

What is MDF?

Medium Density Fibre Board – better known as MDF is a composite panel made from wood fibres that have been reconstituted into a very finely structured homogeneous board using urea formaldehyde resins as binders. MDF is widely recognised as an ideal substrate for panel products.

Its properties include a smooth surface, uniform density and thickness, dimensional stability, strength stiffness, flatness, screw holding strength, plus machinability – these all contribute to the ease of fabrication and versatility of MDF panel products. MDF panels are typically used in applications such as table tops, countertops, shop fittings, kitchen and bathroom cabinets, plus stereo and television cabinets.

The versatility of MDF board is never better expressed than when the flat panels are fashioned into an elegantly shaped pattern found in routed kitchen door profiles and bull nosed edges.

However, the very act of routing out the profiles and edges can cause coating problems in the form of edge cracking. This problem happens infrequently but when it does, the repair required is expensive for the manufacturer and intrusive for the customer.

MDF board consists mainly of wood fibres, which like all wood based materials are hydroscopic. As a result, they are capable of absorbing or losing moisture and will maintain a moisture content approximately in equilibrium with the humidity in the surrounding air.

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Temperature & Atmospheric Conditions

MDF board will change dimensionally in response to an increase or decrease in atmospheric moisture content. This is especially so at the routed edges or profiles where less dense core of the board is exposed to the atmosphere.

MDF board manufacturers despatch board with low moisture content (average 8%) and make recommendations regarding maintaining dimensional board stability to and during use in the joinery workshop. normally the minimum conditioning interval required before use is 24 hours. It is important that these are followed because for every 1% change in the moisture content of the board results in 0.35% change in thickness of the MDF panel.

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When to Coat MDF

MDF should only be coated when it is in a state of dimensional stability. if the board is coated any other time, moisture will slowly permeate through the coating, either inward if the board has been stored in a dry environment (and cause swelling), or outwards if the board has been stored in a damp environment (thus shrinking).

As the board shrinks or expands to achieve dimensional stability the coating has to take up this movement, and if excessive, the coating will not be able to cope and cracking along the vulnerable, less dense routed edges and profiles will occur.

To give an indication of how thin the coating film is, imagine the edge of a 20mm MDF board with a moisture meter as moisture is deep in the center if the board. The harder faces of the MDF board board prevent moisture reaching the surface on the faces while the edges will show the moisture content of the board as related to the atmospheric conditions, not moisture contained within the board.

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How to Minimise the Likelihood of Edge Cracking

Cracking – Use MUF instead of MDF Board: MUF is made with melamine modified urea formaldehyde resin to provide greater moisture resistance, and although slightly dearer and heavier than MDF board it goes a long way to preventing moisture uptake in the core of the board. It is the recommended board for kitchen applications in Australia and is the better alternative in highly variable climates.

Use routed profiles that contain no sharp corners: Coatings pull away from sharp corners leaving a thin and more permeable coating film build in these places. All corners should be rounded to prevent this from occurring. Rounded corners will also make the manufacturing process quicker plus prevent sand through of the coating at these critical points that further reduces the finished coating film build.

Use routed board face profiles that have a depth of less than 30% of the thickness of the board: Ensure that the routed profiles cut into the face of the board (as opposed to the edges) do not penetrate to a depth greater than 30% of the thickness of the panel. If there is not sufficient board left beneath the face routed profile the panel will warp with resulting edge cracking in the bottom corner of the routed profile.

Use sharp router blades at the correct cutter speeds: This is to prevent fibre tear at the crucial board edge interface and thus minimising the opening up of the less dense board core to atmospheric conditions, hence Tungsten Carbide Cutters are recommended. Maintenance of correct feed speed of the panels through the router allows the cutters to preform to their optimum efficiency.

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Sand the edges with 240 to 320 grit paper: This will give a smooth finish to the edges and profiles of the board and thus minimise the use of excessive coating film builds as a filler for the routed edges. Excessive coating film builds are less pliable than correct coating film builds and this more susceptible to cracking.

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Seal the edges of the board with MIROTHANE PU 5500: As soon as the routing process is completed and while the boards are still dimensionally stable, seal the edges of the boards with MIROTHANE PU 5500 to prevent moisture movement in or out of the board. If boards edges cannot be sealed immediately store in a suitable environment to maintain dimensional stability. For more information contact your friendly Sales Representative.

Apply a coating system to the boards: As soon as practical and while the board is dimensionally stable, apply a coating system to the boards. This will create a barrier between the ambient atmospheric conditions and the board substrate.

Apply the correct volume of coatings: A reduction in the coating film below manufacturers’ specifications will reduce the ability of the coating to withstand moisture permeation. Alternatively, too high film build will result in a less pliable, crack prone coating.

Use a MIROTHANE TWO PACK Polyurethane Coating System: Two pack polyurethane coatings are better suited to preventing edge cracking problems because of the stronger and more integrated film build which posses a high resistance to permeating moisture, reducing the likelihood of edge cracking.

Once edge cracking has occurred the board will have reached dimensional stability and edge cracking is unlikely to occur again. Repairs are simply made by:

  1. Sanding the damage substrate to a smooth even finish with 240 grit paper.
  2. Filling in the cracks with a suitable and coating compatible filler. Allow to dry and them fair the substrate with 240 grit paper.
  3. Re-apply the selected Mirotone Coating system as per directions on the relevant product data sheet.


Article provided by NZ Mirotone Branch.

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