Rule #1, not all sandpaper is created equal!
No workshop is complete without sandpaper. It is one of the most common items to be found in a professional painter’s toolkit.
Sandpaper is used to smooth a variety of surfaces, as well as remove rust, paint or varnish and can also be used to prepare a surface.
This versatile product comes in different abrasive qualities, called grit, that may range from coarse and rough to almost smooth, as well as some that can be used in water for an extra fine finish on surfaces such as some metals. Depending on the task at hand, deciding on the coarseness of your sandpaper will determine either the success or failure of that task. Here are some tips to help you correctly choose the grit needed for whatever task you are planning.
An easy rule to remember is: the lower the number, the coarser the paper. The opposite also pertains, namely: the higher the number, the finer the grits. Bear in mind that course sandpaper will remove more material from the surface, but it will also leave deep scratches as well. Although using finer sandpaper will make the job take longer, the results will be worth the effort.
Based on the grit count of your sandpaper, you can either achieve a smooth or a rough surface.
To remove tough paint from a painted surface like metal, you will need to use a coarse grade of sandpaper, ranging anywhere from 30 to 80 grits, dependent on the job, Remembering that the lower the number, the coarser the paper. To really speed up the task, you may need extra coarse sandpaper of 16 to 24 grit, for tasks such as the initial stages of sanding hardwood.
For minor issues, like small scratches or imperfections that can be covered by a coat of paint, eg on a drywall, use medium sandpaper of say, 120 grit, to even them out. For any larger defects, you will need to fill in and repair, eg nail holes and cracks. When dry, use a fine grit to smooth the surface. Once again, 120 grit will do the job.
If you want to repair a wooden surface, you can smooth it out using a medium grit sandpaper. Afterwards, switch to a finer grade such as 180 grit and go over the surface again. If necessary, you can even use a higher micrograde grit for a super fine finish.
If, however, you want to put latex-based paint over oil paint, then you need to roughen the surface of the oil paint first to ensure that the latex will adhere. In this instance, sandpaper of 220 grit is perfect to roughen the surface.
Ultimately, no matter what finish you are intending to have, ensuring that you have the right sandpaper for the right task is important, so refer to the grit chart and, if in doubt, speak to your local hardware store for advice.