Though it may seem impossible, there is a way to sand plexiglass if your piece is not the right size for where it's going or if your plexiglass has imperfections, scratches or sharp edges. All you need is a few tools and some patience as you need to tackle the job slowly to ensure that your plexiglass comes out looking its best.
Step 1 - Preparation
Make sure you sand your plexiglass in a clean room on a drop cloth to catch the small pieces of plexiglass that you will be sanding off. Always also wear gloves and plastic goggles to protect your eyes and hands from the plexiglass. Wear a respirator mask as well to avoid inhaling any particles as you work.
Step 2 - Use the Metal File
Start with the metal file before the sandpaper. The file will take off more plexiglass than the sandpaper so you will use the sandpaper after for refinement. Always file in the same direction, starting at one end of the plexiglass edge or scratch you are trying to erase. Start by applying only light pressure and then check to see how much plexiglass you have successfully removed. Remember, it is better to remove too little and go slowly, as you can always keep going and sand off more, but you can't fix it if you sand off too much.
Step 3 - Sand with Sandpaper
After you are reasonably happy with your plexiglass, switch to sandpaper to further smooth the surface. With sandpaper, the higher the number the finer the grade. When working with plexiglass you should use two kinds, first start with sandpaper that is classified as super fine and then move to ultra fine sandpaper. This covers sandpaper from numbers 400 to 1000. Start with a lower number and then move to as high a number as you need for a smooth plexiglass surface. It is not recommended to use a sandpaper that has a lower number than 400. As with the metal file, sand starting at one end and in one direction only. Begin with light pressure and apply more pressure as needed. You may also wish to lubricate your sandpaper before you start sanding. Some people simply use water and some prefer oil or polish.
Step 4 - Apply Plastic Finish
If you are still not satisfied with your plexiglass repair, you could then apply some liquid repair solution made especially for plastics. There are many products on the market that can help smooth cracks or scratches in plexiglass and generally help smooth the surface. Ask a DIY professional working at hardware or home improvement store which particular brand would be best for your project.
Step 5 - Buff with an Electric Buffer
If you are not sure about using a liquid product to finish your plexiglass repair then you can use an electric buffer to make sure the surface is perfectly smooth. Use the buffer just as you would with any other material and gently move it over the piece of plexiglass you want smooth. Do not remove your goggles or gloves at this point as there could still be shavings coming off of the plexiglass.