Plexiglas® acrylic sheet should be handled carefully to avoid scratching its surface. It is easier to avoid scratches than to remove them. Scratched surfaces can be restored to a good finish by a process of polishing and/or sanding. Sawed edges and machined surfaces can also be polished to a high gloss. Where power-operated polishing equipment is not available, it is possible to hand-polish minor scratches from the surface of the sheet. Minor scratches can be polished by rubbing with soft flannel and a good grade of automotive paste wax.
Before sanding, buffing, or polishing, clean the Plexiglas® acrylic sheet carefully. The buffing wheels and compounds should also be free from dirt and grit. Separate buffs should be reserved for Plexiglas® acrylic sheet. They should be cleaned by running the buffing wheel against a hard metal edge to remove hardened tallow, grease or other binders.
The friction of buffing, sanding, or polishing too long or too vigorously in any one spot can generate enough heat to soften or “burn” the acrylic surface, resulting in visual distortion and possibly discoloration. To avoid this, keep the Plexiglas® acrylic sheet constantly in motion relative to the wheel. Use light pressure, and change the direction of buffing often. Air-cooled buffing wheels are often used to help reduce heat of friction.
Polishing techniques vary with the equipment available and the size or shape of the acrylic parts being polished.
Sanding and buffing cause thickness variations in the scratched area of the Plexiglas® acrylic sheet. If skillfully done, these operations cause only minor optical distortions that should not be objectionable for most applications. In critical sections, even minor distortions may not be acceptable. Such sections, even though scratched, should not be sanded or buffed. They should be simply washed and waxed. However, applications that must be sanded should be sanded by hand, using progressively finer grit paper up to grits of 12,000. These grits are available as cushioned abrasive cloth.
If there is a scratch in the Plexiglas® acrylic sheet, it should not be sanded unless the surface imperfections are too deep to be removed by light buffing and the resultant optical distortion can be tolerated. The way to tell if sanding is necessary is to rub your fingernail over the scratch. If it can be felt, then sanding is required. Use the finest sandpaper that will remove the
imperfections. Coarse paper can cause scratches deeper than the original imperfection, and additional finishing operations will be needed.
First try using 600 grit sandpaper wrapped around a rubber-padded sanding block. Sand over the scratch using increasingly larger areas of sanding. If this does not readily remove the scratch, step down to 400 grit. The sanding should be done in directions mutually 30° apart to produce a diamond pattern. After sanding and stepping up to 600 grit, polish the sheet as described below.
Do not use disc or belt sanders dry. The greater danger of heat generation with mechanical sanders makes the use of water or oil coolants doubly desirable. Wet sanders are preferred, but dry orbital sanders can be used with care. Open coat sandpaper should be used, since it does not become clogged as fast as closed coat sandpaper.
The Plexiglas® acrylic sheet should be clean and dry at the start of each buffing operation. Some polishing compounds leave the surface clean after buffing. If these materials are not used, washing should follow the last step in polishing.
If the part has previously been sanded or is deeply scratched, an abrasive-coated wheel is used first. The abrasive is a standard polishing compound composed of very fine alumina or similar abrasive and tallow.
When most of the scratches have been reduced on the first wheel, the Plexiglas® acrylic sheet is buffed on the second wheel charged only with tallow. These first two wheels should be air-ventilated cotton muslin rag wheels and should operate at 3,000 to 4,500 surface feet per minute (SFPM). To calculate:
S F P M e q u a l s 1 ⁄ 4 t h e d i a m e t e r o f b u f f i n g w h e e l i n i n c h e s m u l t i p l i e d
by the spindle speed in rpm.
The Plexiglas® acrylic sheet is next brought to a high polish by a soft, loose buff in which no abrasive or tallow is used. These cleaning buffs should be very loose and should be made of imitation chamois or cotton flannel. The wheels should be 10 to 12 inches in diameter and should run at 3,000 to 4,500 SFPM. A hand-applied coat of wax may be used in place of buffing on the finish wheel, if desired.