Ever wandered into the renovation centre looking for something to patch a hole in the wall? Complicated, isn't it with so many choices. Why so many? Some are made to fill large holes or cracks without shrinking, others are made to give you a fine smooth finish and some try to do a bit of both.
In the first photo you see me sanding ordinary drywall general purpose compound. This is the thin creamy stuff that we always use, together with tape, for joints in the drywall. You cannot put this on thickly or it will crack seriously. It requires several very thin coats, but it sands very easily and to a fine finish. If the house shifts and the joint cracks later, you can reactivate this compound by getting it wet again, even years later, allowing you to remove the first job and do a good job over the same joint again.
Durabond 90 or other 60 or 90 minute products set chemically before drying which prevents shrinkage. But they are almost impossible to sand and they cannot be reactivated with water. So if you need to fill a large hole, start with them but don't let it sag or stand out beyond the wall. Avoid all sanding. Then after it is hard, finish off with a couple of coats of the regular drywall compound.
Sheetrock 20 is a product that is made more light weight, still sets chemically so has less tendency to shrink but is much more sandable, although not quite as easy as with the regular drywall compound. It has become quite popular because it combines a bit of both types and is closer to the home made formulations that professionals tend to use.
Plaster of Paris is really not for use on walls at all as it is neither really strong nor sandable but is more useful for pouring into molds. However some professionals mix a bit of this with the standard drywall compound to accelerate hardening.
The good old powered PolyFiller is much like a plaster of Paris type of compound. It fills holes well but sags easily and is difficult to sand. I don't use it at all any more. The new kid on the block is Poly InstaFill, and I love it. This is a whip cream textured substances that is ultra light. It sets chemically and sands easily. With its lightness, it doesn't sag much at all, allowing the filling of very large holes right up to the surface. The manufacturer claims a one coat application, but in reality there are always some air holes or other imperfections. For a good job you always need a final touch up coat, with either the same material or as I prefer, the standard drywall compound for the final polished coat.
This is one field where brand names do make a great difference. Some stick better, some flow better all while claiming to be the same thing. It a product is not working for you, before you blame yourself, try another brand. My wife actually threw a whole gallon of expensive light weight filler on the garbage heap when she discovered how much better the little bucket that I had was working -- even though it appeared to be the same kind of material. Professional drywallers will swear by their particular brand choice, because it works best for their work technique. I won't buy a no-name brand of drywall compound because here it does make a difference.
Follow this link for detailed videos on Drywall Repairs.