Tip: Cutting Filler

Another one of those things many already know, and others might be pleased to know.

Let's assume you need to slather a layer of epoxy/micro on some convex part like elevator tips. You should try to slop on what may seem like too much micro with the first application, because then you get consistent density and a better finish later. You're not looking forward to the job because it means sanding away a lot of excess material in order to shape the part. Slow and dusty, yes?

Naaa....everything in life is timing.

Epoxy/micro (and polyester filler for that matter) goes through a stage during cure when the filler is quite firm, yet soft and easy to cut, much like dried out cheese. The actual time from mixing depends on the epoxy and hardener you're using. For the popular West System resin with slow hardener, figure five or six hours at 70F....let it go until you can't quite dent it with your thumbnail. It is still way too soft to sand without clogging the paper; what you need is a 12" vixen file. Hold it at an angle and stroke across the part, sort of like an airplane in a 30 to 45 degree sideslip, and you'll be rewarded with nice shaved flakes and curls of excess micro hitting the floor at a rapid rate. Knock it down to a near-profile and shape, then go have a beer and come back tomorrow with the sandpaper.

Try it, you'll like it.

Good timely tip for me! Thanks!

Forgot to mention that the same principle works for glass cloth layups that overlap an edge. Let it cure just to the point where it is no longer sticky, then use a razor blade to slice off the overlap. It will be soft, flexible, and easy to cut. This trim method also eliminates the "glass needles" you have to deal with if you let the cloth go to full cure. Rutan called it knife trim, a standard in moldless composite work.

Down cut only!

DanH said:

Forgot to mention that the same principle works for glass cloth layups that overlap an edge. Let it cure just to the point where it is no longer sticky, then use a razor blade to slice off the overlap. It will be soft, flexible, and easy to cut. This trim method also eliminates the "glass needles" you have to deal with if you let the cloth go to full cure. Rutan called it knife trim, a standard in moldless composite work.

Click to expand...

TIP: be sure to only cut the glass down toward the subsurface. If you go along the edge or (gulp) up and away from the subsurface, you run the risk of weakening or even delaminating the wet glass. Not a good thing

One more useful detail. You may prefer to allow full cure and then rip off excess micro with a power sander, air or electric, which is fine. The classic oscillating sander, like a 17" air inline found in every body shop, is not what you want for this task. Plastic fillers (Bondo, Rage, etc) have ingredients designed to make them easy to sand without clogging the sandpaper. Epoxy/micro is different, a bit more gummy, in particular when you sand hard and generate heat. In addition, an oscillating sander doesn't provide as much opportunity for the debris to work it's way out from between the paper and the work surface. The combination means gummy dust quickly builds up between the paper and the surface and you don't cut very fast.

What you want for cutting down micro and/or excess glass fabric layers is a hand-held belt sander. Because a belt moves one way, the dust gets spit out the end and doesn't build. I've been using a cheap 3x18 Black and Decker about 25 years. It is light enough to work one-handed, meaning you can sculpt micro, glass or block foam with ease. Load it with a fresh 80 grit belt. The only problem you'll have now is cutting too fast.