How to get a good finish on MDF
As a manufacturer of MDF skirting boards, we receive plenty of questions regarding how best to paint MDF mouldings – especially from customers who have chosen to purchase un-primed mouldings. This article will highlight complexities of painting MDF and offer solutions.
The Grade of MDF is Important
MDF is made up of wooden fibres, which have been compressed and bonded together with resin under high heat and pressure. Typically these are sandwiched between two smooth, hard surfaces, creating a layered pattern on the edge. The grade of MDF will often dictate how good the finish will be. Generally speaking, the higher the grade of MDF, the higher the density. Denser boards provide smoother finishes as there are more fibres compressed into them to provide a more even surface.
Standard and premium grades are typically at the lower end of the density spectrum with Moisture Resistant and Industrial grades being at the top.
Why Can MDF Be Rough to the Touch?
The outer layer of MDF is very smooth to the touch and takes paint well. It is the inner layer which causes the most problems.
If the smooth surface has been cut into or removed – e.g. cutting the shape into the surface of a skirting board – the soft fibres within will be exposed. If you held a magnifying glass to a cut MDF surface you would see thousands of fibres standing up on edge from where they have been cut. This is what produces the rough texture. If you painted this surface without sanding, the paint would likely dry onto the upstanding fibres and make the surface even rougher. To get a good paint finish it is essential that these are sanded away.
You may feel the surface is smooth to the touch when brushed in one direction, but rough in the opposite direction. This is likely to be because the manufacturer has only sanded/brushed the fibres in one direction. To get a good finish, ensure they are brushed in both directions.
Getting a Good Paint Finish on the Cut Surface of MDF
Timber is an absorbent material, therefore the whole board will need priming, using an MDF primer, in order to seal the surface before a finishing coat can be applied. The softer, inner fibres of MDF are particularly absorbent. This means it will sometimes require three or more coats to get an evenly primed surface. Painting the surface of the soft fibres can also raise the ‘grain’ – or in our case fibres – which will again result in a rough finish, so make sure you sand between coats.
When it comes to physically applying the paint, without doubt the best way to achieve a good finish is to spray. A spray finish will create a uniform coating, absent of brush strokes, and take you less time to achieve (providing you have the space to do it).
The key to a good paint finish is in the grade of board and preparation. Providing the raised fibres are dealt with correctly this is very achievable.