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Pre-finishing surfaces with WEST SYSTEM® Epoxy, before assembling them, is a method pioneered by the Gougeon brothers—and one that I use whenever possible.
by Russell Brown
Working on flat panels laid out on a bench or sawhorses is often faster, easier, and results in a higher-quality finish. These parts and panels are often joined with an epoxy fillet that can be cosmetic, structural, or both. While pre-finishing panels is much easier, what isn’t easy is filleting all the intersections—assembly, sanding, and finishing.
My New Method (Explained)
I recently started using a new method. I apply the fillets and the finish coat in one day, without sanding the fillets or even cleaning up the squeeze-out. It’s not a method for absolutely perfect fillets, and it only really applies to bright finished (not painted) interiors, but remember, no sanding!
Custom Sanding Block
This method starts with sanding the pre-finished surfaces for the width of the fillet only. It sounds hard, and does take some care, but it goes surprisingly quickly with the right tool. The magic tool is a custom sanding block. Cut a strip of firm foam rubber down to your desired width, usually a ¼” or so. The width of the sanding should match the width of the desired fillet (use the fillet stick to determine). Then attach sticky-back sandpaper. It seems there is some leeway with how careful you have to be sanding, because I haven’t been that careful and have still been impressed with the outcome. The part where the most care is needed is keeping the sanding tool in the corner to avoid making scratches “out of bounds”.
Taping Off the Sanded Edge
The next step is taping off to the sanded edge. Taping off at the intersection between the sanded and glossy surface isn’t super easy, but like so many things, care at the beginning can mean far less work in the end. I use a good quality masking tape because the tape will have to be pulled after the fillet is partially cured. The filleting and finish coating, with a partial cure between, have to happen on the same day. This way a chemical bond forms between coats, and there is no need for an additional sanding step. To maximize available time, it is best to sand and tape the day before filleting. The methods I use for filleting are described in detail in the book Epoxy Basics (from PT Watercraft.com).
Priming for the Fillets
Priming for the fillets requires a thin coat of unthickened epoxy applied with a small brush. This step provides better adhesion and makes the application of the fillet easier. Wipe up the excess unthickened epoxy with scraps of paper towel. For applying the thickened epoxy, I use a Ziploc® or cake decorating bag to lay a bead of the thickened epoxy. Applying a bit more than necessary is often an advantage.
Laying Down the Fillets
The first pass with the fillet stick should be with the stick laid back a bit (not at 90 degrees). This will show where more epoxy is needed before making the final pass at nearly 90 degrees. If possible, form the final fillet in one pass—moving continuously from one end to the other.
I used a sharpened “chisel stick” to clean up the bigger areas of squeeze-out. This makes it easier to pull the tape later. Bumps or imperfections can sometimes be smoothed out by pushing on them with a gloved fingertip later in the day, when the epoxy has set up a bit, before finish coating.
The Finish Coating
The finish coating (with 105 Epoxy Resin® and 207 Special Clear Hardener®) is quick and easy because everything is already taped off, and it just takes brushing on a coat of epoxy. I give my disposable brushes a haircut. I cut them to half their width, which means half as many bristles falling out into your finish coat. Cutting a bit off the tip also makes the bristles stiffer, which is better for epoxy.
Because the fillets will be a bit rough compared to sanded fillets, apply the finish coat slightly heavier than usual. Wait 10-20 minutes before pulling the tape so that any excess epoxy has time to run onto the tape.
Final Thoughts
When using this method, the fillets will have a slightly rougher finished surface than a sanded fillet. The edges of the fillet will be noticeable, and of course, it requires good filleting skills. However, when there are miles of filleting ahead, and pre-finishing panels is your technique of choice, this method can help.
