Opinions differ considerably when discussing the ease or difficulty of polishing platinum, but most agree that when a piece is well polished, it tends to stay that way.  The most common mistake in polishing platinum is misjudging the degree of surface defect and selecting an inappropriate corrective procedure. Surface porosity is a condition that needs to be identified and distinguished separately from the cast surface. Whenever porosity is present, the casting should be burnished prior to filing. 

The key to filing is in finding the finest cut necessary to remove the defects in an efficient manner. If porosity is revealed during the filing process, it is best to stop and burnish that area before continuing.  When filing it is important to remember that you should only allow the file to contact the piece on the cutting stroke.  Back-dragging the file across the piece can be counterproductive to the filing action. The best quality files available will have machined cutting surfaces. Lubricating the files prior to and during the filing process will help reduce the rate of clogging. 

The platinum polishing process requires that the surface be repeatedly abraded with finer and finer grits of abrasives until the subsequent scratches are so fine that they appear
non-existent.  Some of the most common abrasives include emery, silicon oxide, tripoli, diamond, rouge, and many others. All of these abrasives have advantages and disadvantages in use. The most important aspect is the particle size. In natural abrasives, particle size can vary considerably. This problem is not as apparent with synthetic abrasives.  It is best to stick with one standard when selecting your abrasives. 

Just as there is a variety of abrasives on the market, there is an equal variety of support mediums. The most common include paper, plastic mesh, wheel, disc, and others. The bonding technique, as well as the medium, will also affect the effective cutting depth of the exposed particle.  The deeper the particle is imbedded in the medium, the shallower the cut required.  You can improve the effectiveness of the polishing operation by operating in the optimum working speed range for the grit being used.  This is measured in distance over time.

One step that should never be overlooked in polishing platinum is the necessity of cleaning the coarser compound from the piece prior to moving to a finer one. 
Cross-contamination is one of the biggest problems encountered during the polishing process. If the white "compo" compounds are being used, be sure that the proper grit has been selected.  Using a finer grit than is required will generally result in premature flogging of the buff.  Buffing wheels come in a variety of styles and sizes. With the standard polishing motor, 8" to 10" buffs should be used to obtain the optimum working speed. Choices should be confined to the stitched buffs, either treated or non-treated. Any of the standard matte finishes used elsewhere may be applied to platinum.  The only difference is that it is best to finish the piece to the point where the surface is one grade finer than the desired finish.  This will allow the finish to be applied, usually in a single pass, thereby reducing the chance of clogging. In addition, the finish will generally appear more uniform.  Variety can be added to the finish with areas of bright finishes contrasting with matte surfaces.

Another point to consider is polishing the platinum pieces prior to assembly.  This is possible due to platinum's resistance to tarnishing.  If you intend to use a platinum sheet for
die-struck product, it is better to polish the sheet prior to striking.  Platinum can be
tumble-finished using processes similar to those of gold, but it is best to be very selective in what is chosen to be tumbled.  Generally, pieces with smooth, rounded shapes are best. It is best to avoid tumbling pieces with intricate details or sharp edges as these may be rounded off.  Pieces that are gold and platinum combinations should never be tumbled. Abrasives that break down during tumbling are preferred for platinum.  You should follow the same generally guidelines when using the cutdown media as in polishing. Start with a coarser grit and progress to a finer one. Stainless steel shot can be used for the burnishing process in the tumbling operation.

Here it is important to consider the amount of impingement that is being applied to the piece, the amplitude of the vibratory tumblers, as well as the degree of impingement imparted on a piece.  If a barrel tumbler is being used, reduce the rotational speed to cut down on the degree of impingement.  Adding a polishing powder to the burnishing process will help improve the overall finish. The time cycles in polishing platinum are going to be a matter of trial and error in order to produce the best results.  But once you have established what works best in your machines, the process should be highly repeatable.  As a general rule, the cycle times for platinum will be longer than those for gold and silver.
 

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Platinum Finishing
Gregg Todd
Stuller Settings

This is an abbreviated version of the original work. For full technical details, please consult the original paper.