Circular Mottling

Circular mottling is one decorative finish on aluminium castings.

Bugattis have three types of decorative finish, with the dashboard and firewall and Shell petrol tankers being finished with circular mottling.

Do not be tempted to use valve grinding paste as it hardens to concrete during polishing and cannot be removed without damaging your fine finish.

Use a champagne cork pushed into a tubular tool with a shank for mounting in a drill chuck and coarse sand of about 46 grit as the abrasive- use washed sand as beach sand contains salt and will corrode the surface soon after the job is done.

Mount your aluminium panel onto a sheet of chipboard, which is usually very flat unless stored against a wall in damp conditions. Fit all the disks previously cut out for instruments and switches or make new ones if you threw them out earlier, and hold them in place with a wood screw.

Do the polishing on a drill press and definitely not a milling machine, as the latter is too precise in pitch and the finish does not look right.

Draw horizontal lines in soft pencil with vertical pitch a tiny bit more than half the diameter of the cork. Start at the left hand with the first circle being partly on a piece of scrap aluminium the same thickness as the panel, then make the second and subsequent circles just over half the diameter of the cork to the right as judged by eye.

The correct and accurate vertical spacing can be assured by clamping a strip of wood to the drill press table and using it as a guide while sliding the panel and chipboard support along. Eye-judged horizontal pitch spacings look good.

Sprinkle the sand over the panel and use only light pressure and slow speed on the drill.

After a few circles a projection at the centre of the grinding face of the cork will form and this must be cut off with a very sharp knife or removed by dragging a piece of plywood with coarse sand paper glued on under the cork with a very light pressure  on the drill.

Do not be tempted to use any type of clear varnish on the finished surface as certainly a white oxide will form over time and the varnish cannot be removed to polish it off. Wattyl advice was to grind it off and start again.

Best to use a car wax polish containing carnauba wax and polish with a soft cloth, which you are sure contains no sand nor grit.

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