Trade Secrets: Basic Techniques of Surface Preparation for Wooden Boatbuilding and Repair
Whether you plan on bonding, laminating, filleting, fairing or applying fabrics, the success of the application depends not only on the strength of the epoxy, but also on how well the epoxy adheres to the surface to which it is being applied. Here are some key steps for surface preparation that can play a key part.
“Surface preparation is king. I follow the mantra that there are three stages of surface preparation when working with epoxy: 1) surface preparation, 2) surface preparation and finally 3) surface preparation… you can never do too much!” Richard, Wessex Resins & Adhesives.
Special preparation is required for specific substrates, please refer to the manual for more guidance on the materials of your project including aluminium, steel, polyester and hardwoods.
Surfaces must be free of any contaminants such as grease, oil, wax or mould release. Recommended products for cleaning contaminated surfaces include WEST SYSTEM 850 cleaning solvent, acetone or a silicone or wax remover. Ensure you wipe the surface with clean white paper towels before the solvent dries. Clean surfaces before sanding to avoid sanding the contaminant into the surface. CAUTION! Follow all safety precautions when working with solvents. This technique is particularly useful when working on materials such as teak/oily woods.
All bonding surfaces must be as dry as possible for good adhesion. If you are short on time, to accelerate the drying, you can warm the bonding surface with hot air guns, hair dryers or heat lamps to aid in the drying process. Use fans to move the air in confined or enclosed spaces. Condensation may occur when working outdoors or whenever the temperature changes in the working environment so look out for this.
For teak/oily woods, make sure the solvent has evaporated before coating, but apply the epoxy within 15 minutes of the solvent wipe.
Sanding is labour intensive, but necessary
Sand hardwoods and non-porous surfaces thoroughly to obtain an abraded surface. 80-grit aluminium oxide paper should be used to provide a good mechanical key for the epoxy. Make sure the surface to be bonded is in solid state before you begin sanding. It is important to remove any flaking, chalking, blistering, or old coating before sanding. Ensure you remove all dust after sanding. If you are removing old varnish, this can be labour-intensive, so using a heat gun or paint stripper can be considered more time efficient. We recommend using a marine abrasive in particular for effectiveness.
Removing amine blush
Amine blush is a by-product of the epoxy curing process that may appear as a wax-like film on epoxy surfaces during the final cure phase. Amine blush can occur more so in damper, colder working environments. A simple remedy to treat the area affected is to wash the surface thoroughly with 855 Cleaning Solution and then wash with clean water and an abrasive pad. Dry the surface with plain white paper towels to remove the dissolved blush before it dries on the surface. After washing with the abrasive pad, the surface should appear dull. Sand any remaining glossy areas with 80-grit sandpaper.
Thankfully amine blush is water soluble and can easily be removed but can clog sandpaper and inhibit subsequent bonding if not removed, so it is important to ensure you remove amine blush early on when discovered.
When is the right time to sand?
Using your thumbnail check to see if an impression can be made. If the result equates to an impression, then it is not hard enough to sand. If you notice the surface feels wavy, then allow the epoxy to cure fully and then proceed with a wash before sanding.
Looking for more information?
If you are endeavouring to build, repair or renovate a marine structure with WEST SYSTEM epoxy visit the WEST SYSTEM user manual for more information on surface preparation. Click here.
We hope you enjoyed this article. Keep an eye out for our next article on structural bonding with epoxy.
See you next time!