Ask the Expert Question-and-Answer Archive

by Ron Joseph

March, 2004

Paint Defect on Ply-Wood and MDF

Q. We have a small professional spray shop and custom paint ply, solid and mdf wood products for clients. However, we always have a problem with the paint splitting and cracking after a short while. Why is this happening? Is there a specific paint to use? Cat- pre-cat- none? Latex vs oil, etc. Any help would be greatly appreciated. We always use a sealer after the painting process.

A. There can be a number of reasons that the coatings are cracking after application, one reason can be high moisture levels. It sounds as if the substrate may not be as stable as it should be, such as excessive moisture in the MDF or Ply wood. A higher moisture content can cause the substrate (wood product) to expand and contract to a greater degree, and after a period of time it will tend to migrate out causing the finish to crack and lift. The optimum range of moisture content for wood is between 6% and 8%.

This is an area where your coatings supplier should be able to assist you in finding the root of the problem. Your supplier should be able to measure the moisture content in the wood products with the use of moisture meter. These are specifically designed to measure the percent moisture content in wood products.

There are two types of moisture meters, pin type and pin-less. The pin type meters measure the electrical resistance between two pins. The electrical resistance changes with the water content of the wood. If moisture varies within the small measuring area, the highest value is indicated.

Pin type meters give precise readings and indicate the highest moisture in the area between the pins. These meters can also measure differences between core and surface moisture by using insulated pins that measure only at the depth they are driven to. This is essential for the detection of unevenly dried lumber. Be aware that there will be pin holes left in the wood after measurement.

Pin-less meters use electromagnetic wave technology, which measures density in a three-dimensional field underneath the instrument. The density or specific gravity changes with the water content of the wood. If moisture varies within the segment , the average moisture content is indicated.

Pin-less meters scan large areas quickly and is a good indicator for water pockets and areas of higher moisture levels across a board. They indicate an average moisture level in a three-dimensional measuring field below the measuring pad. These type of meters can not show the differences between surface and core moisture. They do not leave pin holes.

You should also be certain that you are using the proper coating material for the type of wood product and the type of finish you are attempting to achieve. Again speak with your supplier, and inquire as to the recommended products. This also may be a good time to investigate other coatings suppliers and there products.

If you cannot solve the problem through your vendors, we would be happy to assist you on a consulitng basis. Please get back to us if we can assist ytou further.