Platinum Finishing 2 – Procedures and Supplies

In the last issue I wrote about safety issues concerning polishing platinum. In this issue we will continue the discussion with notes on Platinum Polishing Procedures and Supplies.

The entire process of finishing any material is about first defining the shape and then smoothing the surface. The last stage of finishing is to bring the item to the desired level of surface finish. Your last phase may be to apply a texture such as sandblasting, or a high gloss, but the impact of the final finish rests on the prep work that has preceded it. A moderate cutting medium follows a coarse medium and so forth until the desired surface finish is achieved. This applies to finishing any material.

In polishing platinum, as in cutting stones, proper progression of cutters and abrasives is critical. If you should find that you have reached the level of pre-polish and find that there are uneven surfaces, file marks or other imperfections in some part of your surface, you'll need to go back as many steps as necessary to remedy the situation before resuming your pre-polish phase. You won't be able to buff out these imperfections with the grade of abrasive that you thought you had graduated to. Gold and silver might let you chase out a surface problem with a pre-polishing compound but platinum will fight you and probably win.

For filing, a cookie sheet that will nearly fill your catch drawer is convenient and a separate cookie sheet can be kept for each metal that you handle. This is handy for identification of the alloy of the trimmings in the pan if you don't clean up right away, as well as helping to segregate your scrap for reclamation.

After filing, sweep any filings off your bench pin into your catch tray with a stiff, short bristle brush. Put your catch tray (cookie sheet) aside to open your catch drawer for sanding.

Files

Sharp files and cutting tools will cut where you direct them to cut. Dull tools will wander or drift. If you try to do heavy material removal with a moderate or fine cutting medium, youmight get it done butyouwill lose accuracy. You'll blurthe area that you're working due to prolonged exposure. You'll waste time and the net result will suffer. You'll be traveling in reverse. A file that is as aggressive as the job requires will allow you to focus on efficiently removing material - getting it done crisply and moving on to the next step.

Saw Blades

The correct size saw blade will crisply and quickly cut out patterns from sheet stock or closely trim the gates off of a cast part. A fine saw blade may also be used as a file to clean up the initial saw cut in confined areas or to clean out and to smooth passages in a casting. Working the side ofthe blade against the edge that you want to refine as if it were a file does this. After the utility of a saw blade has been exhausted, a file is typically used to follow up in removing surface irregularities, marks from the teeth of a saw and refining a surface in preparation for "finishing". Again, a coarse file may be needed to remove initial mass - followed by a moderate cut file and then a fine cut file to blend and smooth your work. #6 files are as small as I use in my work. Saws and files are both multiple-tooth cutting instruments and are likely to leave behind surfaces that reflect the pattern of their teeth.

Additional Cutters

After the saw blade and the file, the next rigid cutter is a single cutting edge such as a graver, knife (scalpel or #11 utility) blade or a carbide scraper. These can be used to further define and smooth an area. A sharp single edge can make a bright cut leaving the resulting edge either ready to polish or without the need to polish.

Abrasive Sheets

There are lots of abrasive papers, films and such on the market these days. The products listed below are what I use. You are free to use any product that proves to be effective for you.

I'll work on the inside of a ring before tackling the outer surfaces with abrasive sheets on a split mandrel. I like to cut strips off of "Norton 3X" sheets for use on a split mandrel in the flex shaft. So far, I have found the 3X sheets available only as fine as 400 grit. I use Norton "waterproof" brand for my 600 grit needs. I'll use 220 or 320 grit and then move on to 400 and 600 grit strips progressively. There are also abrasive barrels in various grits that fit onto mandrels with rubber grips and cones of abrasive paper that fit on tapered mandrels on the large polishing lathe. I use these from time to time but, as most of my rings feature rounded interiors without hard edges, I find that the split mandrels work well for me.

Sanding Sticks

Outside surfaces
 are sanded with the
same progression of
 abrasive sheets listed 
above but for the outside surfaces, I wrap 
my abrasive sheets
 around sanding sticks.
To do this, I'll lay out
 a sheet of abrasive 
paper, face down, facing away from me the long way and then I'll align my sanding stick inside of one of the short sides. I'll score a line in the sheet with a scribe and fold the stick into the sheet at the scribed line. After the first fold is made, 1'11 scribe the sheet again in preparation for the next fold.

The reason for scribing these lines before folding is so that the corners of my stick will be fairly square and crisp and can be used to refine intersections and meeting surfaces in my metal work. If the paper was just rolled on to the sticks without scoring, I'd end up with rounded corners and flats that are more arced than "flat". I'd have less control over the precision of my cutting with no options to cut into square or angular intersections in the work piece. Continue to scribe and fold until the sheet is completely wrapped on your stick. Staple the sheet to the narrow side of your stick and trim off the extra bit on the end that is too narrow for another fold. Save the trimmed strip of abrasive paper for your split mandrel work. These sticks are best used on larger, broader surfaces. Use silicone, acrylic or rubber bound abrasive wheels shaped to suit the contours of your work - to cut into tight intersections and to clean up small lines, grooves and details.

Abrasive sticks and sponge-backed abrasive products are available in your tool catalogs. There are also fingernail "files" made in various grits that have been recommended by others. Try them all. I like the long stroke and utility of the full width of a sheet of sandpaper to work with.

Brush off your bench pin and clean your work and hands before moving to the next, finer grit abrasive.

See also:

• Platinum Finishing Techniques - Safety
• Platinum Finishing Techniques - Notes and Procedures

By Mark Morrell © Bench Magazine 2005 Winter

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