Taping the joints to create a seamless finish is the challenge of working with drywall. Joint compounds are chosen according to the installer's skill level and the allowable time frame for the work. Toppings and all-purpose joint compounds have fundamental differences that affect the amount of time a drywall finishing job takes, but each one will produce satisfactory results for the finish work, as long as you take a meticulous and patient approach.
Gypsum board, commonly referred to as drywall, is placed on the framing members of an interior wall in 4-foot-by-8-foot panels, and fastened with screws. A taping process fills in the gaps between the joints of the panels, resulting in a uniform appearance when the drywall is painted. Drywall tape is dragged through jointing compound, topped with more compound and then sanded to a smooth finish. The consistency and skilful application of joint compound is critical to the success of the job and the appearance of the finished wall.
Joint compounds are classified as two types -- setting compound and drying compound. Setting compound cures with a chemical reaction, and drying compound hardens by evaporation. Setting compound is available in a coarse texture for the tape coat and a thinner, finer texture for the fill and finish coats. The latter is called the topping compound. Setting compound is available in powder form and has to be mixed for use. It hardens quickly with little shrinkage, making it useful for filling large holes and gaps.
All-Purpose Joint Compound
All-purpose joint compound is a drying type, also available in powder form but typically used in a ready-mix version. All-purpose compound is suitable for use as a tape coat when it's mixed at full strength, and also for top layers when the mix is thinned for workability and easier sanding. A longer period between coats is required for drying compounds than the setting types -- 24 hours for all-purpose joint compound compared with 45 minutes for the taping and topping setting compounds.
Difference Between Topping and All-Purpose Compounds
Topping compounds have a fluid consistency that's easy to feather and sand to a fine finish for the final layer, making them the preferred option for professionals; but because they harden so quickly, it's difficult for novices to sand them properly. All-purpose joint compounds are recommended as the easiest for the amateur to sand to a quality finish without rushing through the job. All-purpose ready-mix joint compound eliminates the need to mix to the correct consistency as topping compounds require.