I had to cut out a square piece of drywall recently for a repair and neatly replaced by fastening the cutout piece into plywood strips, as I show at http://www.drywallinfo.com/holerepair.html. My regular tools were stored away, so I thought I would try using a plastic taping knife my inlaw bought me as part of a putty knife kit. I thought to myself “I know how to tape and finish pretty well – I should be able to handle this job with this plastic knife OK”. Wow, was I wrong! The first coat went on somewhat OK, but then the whole job sort of turned ugly! This plastic knife can not, at all, be used to knock down the surface after a coat is put on. You can try all you want to “level out” the surface, but the wimpy plastic glides over the top or the plastic itself gets worn down. I did the best I could, and then put another coat on. The result is shown below. What a mess! Now at this point, when I was a beginner in drywall finishing, I would have gotten out the sanding block and gone to town, turning bumps and ridges into hills and valleys, and creating a big pile of drywall dust. But I now know better! Instead of sanding, I got my regular tools – metal taping knives of 4″ and 12″ width. I started with the 4″ knife, using both hands to knock the surface down, as shown in the photo. What this did was make my surface level. It did not get rid of all imperfections, but rather it gave me a surface where a wide thin coat could fill in any imperfections and give me a fairly level, filled-in surface.
And you can see in the bottom photo how I coat the level surface with a few 12″ wide fairly thin coats. After coating, I again knock down the surface and then coat with very light thinner coats applied with either my 12″ or my 4″ taping knives. As I get toward the end of the process, the coats get lighter and more thinned out. To get a perfect result, I use a trouble light to inspect as I touch up the final product. With this method, sanding is hardly needed, and in fact I have skipped sanding all together at times. But if you want, you can lightly sand after all coats are on with 220 grit using a circular motion and a sanding block.