Re: Basic Drywall Question.
Drywall from 35 years ago was the same thickness as it is today. The difference you're seeing could be any number of things.
1 - taped seams and corners are going to be thicker because of a build up of the original taping finish.
2 - Texture adds thickness, as does "no texture", which means that the entire surface has been floated with drywall compound.
3 - The original material may be 5/8, which is most commonly found on fire walls and ceilings.
As jkirk said, you can fur up the new to the same height as the old, or use thicker material or, if the difference isn't off that much, just float it out when you tape the joints.
Proper taping procedure depends on the tape you're using. If mesh tape, apply the tape to clean, bare drywall and press it into place with your hand or knife edge. First coat with regular joint compound, subsequent coats can be either joint or topping compound. If using paper tape, then a thin layer of joint compound is applied to the joint. Set the tape into the wet mud and lay it off with a knife to smooth and press it into place. Be careful not to press too hard and squish all the mud out or the tape will fail. You can immediately follow setting the tape with another coat of joint compound. Once dry, continue with topping compound until the desired finish level is achieved. Texture is applied once the area has been sanded smooth.
Using a good primer such as blue label Zinsser, apply two coats to the patched areas and one coat to the entire wall. Follow that with two coats of a good quality paint such as Kelly Moore or Sherwin Williams.
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