William,
I think the best heat source for separating the Araldite skin joints is a
flat silicone heat blanket, perhaps an inch wide by a foot or two long (see
www.mcmaster.com, part number 35765K154 for an example of what I am talking
about). A fiberglass tape heater or Kapton heater would work also. With a
contact heater, you can keep the heater temperature just slightly above the
desired temperature of the joint, leaving it there for a few minutes, until
the Araldite has softened. The hot air from a heat gun must be much hotter
than the ultimate temperature of the joint, and it will rapidly overheat the
thin fiberglass/foam sandwich adjacent to the solid fiberglass/epoxy joint,
probably damaging it before the joint can get up to temperature. When the
Araldite has softened, it should be possible to slide a putty knife between
the skin and the rib or spar. The best place to start is at a corner. The
good news (in my experience) is that the glass transition temperature of the
Araldite is lower than that of the laminating resin, so the joint should
soften before the skin delaminates.
For best control of the heating rate, run the heater from a variable
autotransformer (Variac) or a lamp dimmer. Putting a thermocouple between
the heater and the skin lets you know how the heating is progressing. While
I have not tried to separate fiberglass joints this way, I have used this
heating setup to deform the upper "shelf" portion of the fuel tank, as it
was interfering with the wing spar, and preventing rigging of the wings (a
problem with the early XS construction manuals). The temperature range
between hard (too cold) and runny (too hot) for the plastic tank is narrow
enough to require fairly delicate temperature control.
Good luck!
Dave DeFord
N135TD (XS monowheel)
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