how to sand plasterboard joins
Sanding is perhaps the most important step in plastering. The end result is more often than not effected by the quality of sanding.
Using the correct sandpaper and sanding float is very important. All through the tips on stopping up joins, I have stressed scraping back excess base coat. Here is the reason why. Base coat is harder than top coat, so when sanding, if you try to sand out a bump of base coat, all you succeed in doing is sanding off all the top coat around the bump, making the bump worse not better! If you get this situation, it is better to scrape off the offending base coat and apply another coat of top coat over the area.
The purpose of sanding is to make the join invisible. Provided the join has been troweled up correctly very little sanding should be needed. The most important part of sanding, is to blend the edge of the join into the surrounding surface. When sanding the edge use the end of the block, slanted toward the center of the join. Sand the edges first, then with a circular motion sand through the center of the join, holding the block flat to the work. Sand off only what is needed to provide a flat join. Take care at intersecting joins that the edge of the sand paper doesn't dig in creating a line in your work. Tear off the corners of the sand paper that turn over the end of the block. This will prevent them digging into the opposite side of an internal corner.
When sanding an internal corner it is better to use a small foam sanding block, but take care not to dig into the opposite side. Often shrinkage in the top coat will create what looks like a crack in an internal corner. The sharp edge of the small foam sanding blocks are great to sand out this crack leaving you with a square corner.