The best material for repairs depends on the extent of the damage to be repaired and the type of material being repaired. A general rule is to make repairs with like materials, but drywall compound is so versatile and easy to work with that it can be used for many repairs to plaster surfaces. Unlike plaster, it does not have much dimensional stability of its own, so it must be supported with underlying structure for large repairs. It also expands and contracts in response to temperature and humidity changes differently than plaster does, so cracks often appear where the two unlike materials are joined together.
Spackle is not suitable for any kind of repairs. It is primarily a consumer product attractive because it can be painted over instantly. Professionals seldom use it because it will not work for anything but filling small holes such as those made by picture frame hooks. It has no dimensional stability or strength, is difficult to tool to a smooth surface, and cannot be sanded or finished nicely. Compared to other repair option materials, like drywall compound, it is expensive.
Plaster is sold in powder form and mixed with water on demand. It is dimensionally stable when cured, is extremely strong and hard, and stands up well to temperature and humidity changes. Its drawbacks are that it can be brittle, and if moisture gets into it, hidden failures can cause it to start disintegrating, flexing or sagging, or falling off from its supporting structure in chunks. It is the preferred choice for repairing existing plaster walls and ceilings, since it matches the expansion and contraction characteristics of the existing material. It is also an excellent option for casting replacements or replications for historic moulding, ceiling medallions, other trim and architectural elements. Even material costs for things like Venetian marble plasters remain affordable, but finding experienced plasterers is increasingly difficult and potentially expensive.
Drywall compound is an excellent repair material for most modern construction applications that probably have drywall walls and ceilings, and it can be used for repairing minor cracks and surface imperfections in plaster. It is easy to tool smoothly and sand, either with sandpaper or using a wet method with water and sponge. Drywall compound does not have a lot of dimensional stability of its own, and it shrinks when drying. It must have adequate support behind it or be built up in layers, even for smaller holes. Actual drywall is cut and screwed in place, and seams are taped to provide support for drywall compound when making large repairs.
Drywall compound is inexpensive and comes in different types. Most sold today is lightweight, low dust and comes premixed in plastic containers ranging from quart size to 5-gallon buckets. "Hot mud," as setting-type drywall compound is known, comes in powder form like plaster and is mixed with water as needed. It is often the better choice for repairs, because the thickness of the material can be adjusted and the drying time is more predictable. Hot mud is sold according to its approximate cure time in formulas that cure from five to 120 minutes. Curing time starts the second moisture hits the powder, so quicker curing times might not be the best choice for novices not accustomed to working with the material.
Drywall compound is versatile and can be modified easily with additives like sand or polystyrene pellets and sprayed or rolled on to create texture. It can be tooled with different trowels, brushes and texturing tools to simulate or match different surfaces or disguise cracked walls and ceilings where plaster sections have separated. Some restorers even mix drywall compound to a consistency that flows through cake decorating tips for making repairs to architectural details.