Fiberglass

Anette Aalling <aaa [at] konservering__vejleamt__dk> writes

>Does anyone have any experience with glass fibre art? I have a
>problem with the surface of the laminated fibre glass--the upper
>layer called the gel-coating. ....

I have had some experience with art works made of fiberglass
reinforced plastics.  Polyester resin is the most common binder,
though epoxy and many others are possible.  Typically they are of
the 'thermosetting' type--i.e., cross-linked and inherently
irreversible.

>... The gel-coat, which contains the
>colour pigments, looks milky in the upper surface. It is not
>homogeneous whitening, but it looks striped as if the whitening are
>done by a brush. ...

Does the pattern of the whitening have any correspondence with the
pattern of the colors (I suspect that it does not)? The typical
gel-coat is polyester resin with additives to make it more
thixotropic as well as colorants--it is the first thick coat applied
to the inside of a mold, or the top coat of a laminate built over a
core (such as a surfboard).  Is this what is your art work made of?

>... The surface does not have microscopic cracks and it
>seems to be in good conditions in the microscope. ...

I think that synthetic polymer resins tend to degrade rather
evenly--the surface just crumbles away.  This may not be obvious
under a microscope.  Exposure to sunlight seems to accelerate the
degradation.  Cracks tend to occur more often as a result of
mechanical stress.

>... I assume that the
>colour differences are coursed by additives of the laminated fibre
>glass, which migrates to the top of the surface.

There may be some additives such as plasticizers that do migrate to
the surface, but they may not be the only or primary cause of the
milky haze or surface whitening.  My best guess is that whitening is
caused by the degradation products of the polymer resin itself, and
that it is irreversible.

>It is possible to grind and polish the surface to the right colour
>again, but the gel-coat is only 0.6-0-8 mm, so I would like to avoid
>this method.

Yes, This removes the white layer and exposes a new surface which
continues to degrade. I try to avoid or minimize this method too,
but it does restore the original appearance for a while.

>Does anyone have experience in these matters? Do you know if the
>white layer will come again? Has anyone tried to regenerate a
>gel-coat with success? And if there are cracks or loss of surface in
>the art object, which material did you use to fill up and retouch
>the damage?

The white layer (blanching) will probably return again, perhaps
sooner if the surface is unprotected.  I don't think it possible to
"regenerate" the gel-coat, but I have applied coatings of wax and
other plastic resin.  As, for filling losses, I have used the most
similar resins and pigments to the original materials that were then
obtainable.  If you wish for more detail, feel free to contact me
directly.  Good Luck,

Jim Grant, Conservator in private practice


                                  ***
                  Conservation DistList Instance 15:17
                 Distributed: Tuesday, August 14, 2001
                       Message Id: cdl-15-17-002
                                  ***