Prepolish
The prepolish step was done with the 9 micron grit with felt polishing
pads. Rough and fine grinding was done with hard surface to hard surface.
This results in the grit 'rolling' over the surfaces. The result is the
piting of the surface. Kind of like a boulder rolling over the landscape.
Each time it hits, it causes a divot. To stop this divot action, we shifted
to using felt polishing pads with the new tool shown below. The pads
are in the shape of those rubber no-slip things some people put in the
bottom of their bathtubs! The new tool was cast from the shape of
the rough ground mirror. While not an exact fit, we hoped it was
close enough.
What will happen now is that the grit will become embeded within the
pads. Now the grit rather than playing boulder and rolling and forming
pits, the grit will be embedded in the pads and perform more like sandpaper.
Once the surface becomes 'smooth' enough, we will start the true polishing
of the mirrors.
The prepolish tool was carefully watched during this phase since no
one in the group had ever used these polishing pads before. We had
several concerns.
-
The tool form was made from the rough ground mirror. Was the shape
close enough to the target shape to correctly polish the entire surface
of the mirror?
-
Would the pads really stay stuck to the plaster tool? Plaster is
a difficult surface for adhesives to stick to. If the pads came loose
between the mirror and tool, bad things could happen.
-
Would they really do anything useful?
They seem to have done well however. Members watched while the grinding
continured and the glass got clearer. For the final part of this
stage, we used polishing rouge on the felt pads rather than the 9 micron
grit.
While we were finishing this, we prepared the pitch
lap...