What are the differences between wood repair compounds?
Compound failure can accelerate wood decay
There are 3 basic material types on the market today:
Cemenitious, Polyester resins, and Epoxies.
Each material has it's place depending on it's application and expected long-term durability.
Cemenitious:
These materials are 1 part and come pre-mixed or in a dry powder form to be mixed and applied by the user. These materials are normally packaged in tubes or cans. Ceminitious patching materials can either be water based or solvent base.
These materials will shrink and sometimes crack as they dry and harden. They are generally easy to sand and often require a second application if the volume of the repair is larger than a bean size ( 9 cubic mm ). These materials are good for shallow surface gauges or small nail holes and perform best in interior applications where there is little or no movement in the wood.
Repairs to outside corners or areas subject to traffic tend to perform poorly over time because they lack a strong bond into the wood fibers.
1 part repair compounds are usually suitable for small surface repairs in low traffic areas
Polyester Resins
A Polyester Resin is a paste like material which will cure to a solid when the hardener is added. The hardener is a MEKP (Methyl Ethyl Ketone Peroxide) and is added to cure, or harden the resin. They are usually dispensed from a small tube and have a white cream like consistancy.
Polyester resin with cream hardener
Polyester resins have a very strong odor and should be used in a well ventilated area. These materials are often referred to as auto-body fillers. PR's cure quickly ( 10-15 open time ) depending on temperatures with little shrinkage.
These materials tend to slump and sag before curing on a vertical surface and require multiple build-ups for repairs greater than 9 cubic mm. PR's tend to drag and pull when applying making if difficult to achieve a contoured repair without sanding or carving when cured.
Polyester compounds provide an adequate bond to wood if used where expansion and contraction is kept to a minimum.
Epoxy
Epoxy resins can be engineered to a variety of physical properties usually measured and tested by ASTM standards. Epoxy resins are regarded as excellent adhesives and are used in marine, aerospace, electronic, and construction applications.
Epoxy based patching materials are 2 part components varying in mixing ratios from 1:1 to 20:1 depending on the chemistry. Curing and performance properties can vary and are usually engineered to suit custom applications.
Epoxies are exothermic curing materials, meaning they cure by the heat generated in the chemical reaction of mixing parts A & B. Epoxy curing times are also effected by working temperatures: The hotter it is, the faster it cures. The colder it is, the slower it cures. Another issue to consider is the size of the repair and curing time. Because of the exothermic curing properties of epoxies, small repairs cure slower and large repairs cure faster.
Curing times can vary from 5 minutes to hours depending on the chemistry, working temperatures, and the size of the repair.
For wood repair a high degree of flexural strength ( ASTM D-790 ), and peel strength ( ASTM D-1862 ) are required to provide a high performance bond to wood fibers.
Some epoxy resins can also require thickening agents to increase the viscosity for vertical repairs.
Dura-Fix Slow Cure repair complete ready for paint.
Other considerations are: ease of metering and dispensing, sculpting to desired shape before cure, open time and cure time before sanding and coating. If a natural wood finish is desired some epoxies can easily take a liquid or powder based tint.
We have engineered the Dura-Fix Slow Cure with a high degree of both flexural and peel strength. Dura-Fix Slow Cure has an open time of 30 minutes with an 8 hour cure time.