I have noticed that on a couple of boards that I made from clark foam and glassed with epoxy that after a few months it appears the glass has contacted around the foam compressing it. This is obvious on the bottom because the once flat stringer was now almost 1/8" high. On the deck this leads to glass cracks along the stringer. Has anyone else seen this happen?
I have a kneeboard that I built in 1974 using Clark foam (including a stringer) and epoxy (with uni- and bidirectional glass). It was a project to become familiar with the glass and resins used to build a Vari-EZE. The latter is a two-place, homebuilt, canard-configured aircraft designed by Burt Rutan–the designer of the Voyager (the first aircraft to circle the earth without refueling) and Space Ship One, (first private plane/rocketship to carry a person into space), and a number of other unique aircraft (including the Beechcraft Starship). I don’t remember the brand of resin, but it was purchased from Aircraft Spruce and Specialty (in Fullerton, CA).
Now–30 yrs later–I can’t detect any evidence of the problem you describe. The principal problem that I experienced is a cosmetic one: over time, the “FeatherFill” that was used to provide a lightweight sanding surface over the laminate (instead of using a resin coating) peeled away in a few spots. Other than that, the board is in fine shape, still occasionally used, and presently on loan to a friend whom I’m trying to convert from bodyboarding to both bodyboarding and kneeboarding.
mtb
PS. Anyone interested in buying an unfinished Vari-EZE kit (includes machined parts: brakes, wheels and axles, wing attach fittings, etc.; plans, tires and tubes, cowlings and canopy, landing gear, pusher prop and extension, spinner, some instrumentation (no nav/coms), and possibly a zero-time since rebuild Continental O-200 aircraft engine? Needs glass, a block or two of foam, new epoxy, and various lesser items.
Urethane foam is very sensitive to UV. Most epoxies are not produced with urethane in mind so they don’t generally have the kind of UV absorber needed for urethane. We sell a UV absorber as a resin additive that will solve your problem. Or you can paint the blank white before laminating.
is this UV absorber something in addition to “Additive F”, or is “F” an all-purpose surfacing agent / uv absorber / miracle additive???
Greg…thanks for the response. Do you feel that it is uv light that is causing the foam to shrink under the glass and leave the stringer high. I know the stringer was flat with the foam when the board was glassed. Now it is almost an 1/8" higher than the foam. This board is about a year old.
I doubt UV light was the issue. Did you use an Ultra Light blank and overshape it?
It is very difficult for a Clark Foam blank to shrink due to quality controls that they have implemented over the years, however it can happen by overshaping an Ultralight blank if you are very unlucky.
Clark Foam typically leaves their 2nd and reject blanks in their yard for months of direct UV. They don’t shrink, they just get harder and harder.
Or maybe you didn’t do as good of a job as you thought taken down the stringers.
Ive made quite a few boards out of Epoxy, VE and Iso’s that had no UV inhibitor and have never seen this occur.
Sluggo
I have a theory about what’s going on, based on the nature of the materials, but I’ve never seen this, so it’s only a theory–
Given that epoxy is more resilient than the foam underneath (epoxy bounces back, while foam crushes) what may be happening, is that as the board flexes, the tension side is slowly crushing the foam over the entire bottom of the board… same for the deck when it’s flexed the other way. I suppose this would be most likely to occur with very light foam, and a thinner more flexible lam…EPS would just bounce back, and is therefore a more suitable foam to use with epoxy.
Just a guess…
Wells
I think your guess is spot-on Wells. I’ve seen the exact thing happen even with polyester. I think we’d all be amazed just how much flex occurs with our boards…