Blow molds and many prototype molds are made from aluminum. Aluminum is a very soft material and requires special polishing procedures. A fine finish can be achieved, although it may never be as fine as the finish that is possible on steel. The following is a basic polishing procedure for aluminum. In time and with practice, each polisher will develop his or her own tricks and short cuts.
Prior to Start: Note that a ground finish is finer than a milled finish. You will probably need to start with a 120 grit stone to remove milling marks, while you can usually start with a 320 grit stone to remove grinding marks. Use a criss-cross pattern during all stoning and polishing procedures. Stone or polish the surface in small increments, covering an area no larger than about 1-2 square inches, depending upon the entire surface area to be polished.
Step 1: To remove milling or tool marks, start with a 120 grit stone for coarse tool marks. Since aluminum tends to smear, use ultra soft stones. Most other stones are too hard for aluminum and will load up quickly. Be sure to use a liberal amount of stoning oil, which will act as a vehicle to wash away residue and grit, and also prevent the stone from loading up. Soak the stones for about 10-15 minutes in stoning oil before use, then re-wet the mold surface frequently during stoning. Cleanliness is extremely important. Be sure to clean the mold surface thoroughly between stoning and polishing steps. Keep all stones, polishing compounds and their applicators of like grit in separate containers to prevent contamination of different grits.
Step 2: After the 120 grit ultra soft stone has removed all the milling marks, use a 320 grit ultra soft stone to remove the scratches left by the 120 grit stone. Again, use stoning oil liberally. Continue using the 320 grit stone until all the 120 grit lines are gone and the only lines visible are those of the 320 stone. Stoning in a perpendicular pattern to the previous stoning step will make it easier to determine when the previous stoning step has been removed.
Step 3: Next, use a 400 grit sandpaper (preferably aluminum oxide) to remove the 320 grit stone marks. Then use a 600 grit sandpaper. Apply a liberal amount of stoning oil during both sandpaper operations.
Step 4: Start polishing with diamond compounds. Begin with the 15 micron blue diamond compound by applying it the surface of the mold and working it over the surface using plenty of diamond lapping oil and a cup brush at a speed no greater than 15,000 rpm. This is done for two reasons: 1. to prevent the diamond compound from being thrown away from the surface by centrifugal force; and, 2. because high rpms can generate too much heat on the mold surface causing orange peel. The type of brush depends on the size and shape of the work surface. A small surface may not allow the use of a cup brush. Other brush types are available. Use diamond lapping oil liberally during this process to prevent excess heat from coagulating the diamond. Keep the brush moving and use light hand pressure. Best results are obtained when the brush is used in a criss-cross pattern.
Polish a small section of the mold with the 15 micron blue diamond compound to be sure that you are getting all the 600 grit sandpaper marks out. (If not, you will have to use a finer sandpaper, 900 grit and then proceed to blue diamond compound.) Lessen the amount of hand pressure on the brush to reduce swirl marks.
Step 5: If the finish obtained with the blue diamond compound is acceptable, use an all-metal paste polish1 (with no abrasive) on clean tissue or toilet paper to remove the cloudy residue left by the brush. If you desire a finer finish, use 3 micron yellow diamond compound in the same manner as the blue diamond. If necessary, move to the finest diamond compound, ivory ½ micron. The finish you require will determine how many diamond compound steps are necessary. Use an all-metal paste polish after each diamond step to remove any cloudy marks left by the brushes.
1 Simichrome polish.