First off, as a general rule, there is no reason to go heavier than a medium grit. It can be confusing since there are so many different types of bits to choose from, but I’ll describe a few to get you started.
First is an extra-fine, tapered buffer finisher. This looks like a prepper but the grit is extra-fine, making it useful for prepping the nail before a full set and doing finish work. Some companies call them a tapered smooth. (For finishing you could also use extra-fine sanding bands, which would be thrown away after each use.) The prepper is another useful choice. It looks like the tapered buffer, but it has a medium diamond grit and comes in large and small sizes to fit different size nails. The prepper is used when doing a fill to blend the existing product flush to the natural nail. For filing off acrylic, you could use a carbide barrel or medium diamond barrel to take down the bulk of a nail. The barrel is also good for taking down length.
Keep the bit parallel to the nail. Remember that carbides are the most aggressive and the edge of the barrel can do a lot of damage if not held correctly, so I recommend those just starting out use good-quality diamond bits and a medium round Swiss carbide barrel (the edge is rounded).
I also use a drilling solution, which helps to minimize friction and eliminate dust filings. It is still important to sculpt and shape with your brush even if you are using an electric file to refine the shape.
Having more than one set of bits is important if you are doing clients back-to-back so that you can follow the correct sanitation and disinfection steps. Just putting bits in acetone is not disinfecting; it only helps to remove acrylic from the bit.
Also remember that gels are a softer material to file. Damage to the nail plate can be avoided by using the right bit for the job, holding the bit at the proper angle, and keeping the bit moving to reduce friction. Practice on people close to you or on yourself to truly see how it feels. Remember that pressure causes friction and friction causes heat! Using an electric file without the proper training and practice is not advised. -- Debra Krasniak is an instructor at Cosmotech School of Cosmetology in Portland, Maine.