Carl Pattinson wrote:
>
> I may be stating the obvious that it is not a good idea to leave the cling
> film in place while the resin is curing if you intend to do any more lay
> up's over the cured area. It is much better to use peel ply as this (a)
> create a clean, ready keyed surface from which to work and (b) removes the
> impurities of the resin/hardener reaction which migrate to the surface
> during the curing process.(c) the peel ply draws out excess resin from the
> lay up ensuring a lighter and stronger component.
>
> If peel ply is not used then it becomes necessary the use abrasives and
> solvents to remove the impurities, which in turn weakens the strength of the
> lay up.
That's all basically true, but epoxy won't bond well to a glossy
surface and I once looked at a peel-plied surface under
magnification. It's all glossy on the "mountain" sides. So I browsed
through resin mfr sites, and none I found say peel-ply is better than
abrasion, but seem to treat them equivocally. One mfr does say
abrasion is better; another said a peel-plied surface should be
abraded. But like you say, sanding a non-plied surface can cut into
the fibers. Whether or not that's a problem depends on whether it's
structural, and whether the number of layers of cloth that's there is
just what's needed for design strength or for stiffness also.
I tried testing this out. Non-sanded peel ply and sanded glass seemed
about the same in peel strength. The sanded peel ply was more
tenacious. In each case, once allowed to fully cure, the balance of
test strips tore before breaking the bond.
Regards,
Fred F.
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