The more discriminating decorator (and why shouldn't you be one?) takes their preparations very seriously in order to achieve a blemish-free foundation on which to apply their rolls of Neisha Crosland. Here's a guide to what you can do to smooth your walls ...
Fillers
Small holes, hairline cracks and minor damage can be filled easily using a filler (Polyfilla being the most famous) which is put on with a filling knife or scraper. They are easy to use, soluble in water and available as a dry powder (to be mixed with water) or ready-mixed in a tub or tubes with shaped nozzles, for squirting a neat and even amount of filler around edges and corners. The range includes water-resistant versions for bathrooms and kitchens and flexible gap fillers for use in cracks between two surfaces where movement can occur. The multi-purpose powder form is cheap and ideal for sporadic small holes, but for larger areas where consistency is required the ready-mixed is more convenient. Filler shrinks when it dries so leave it protruding beyond the wall's surface, then sand it flat when dry with fine paper. When filling very deep holes, the filler often shrinks into the hole leaving a "dint", so you may have to fill these twice.
Base coats and smoothers
Base coats consist of a thick paint augmented with fillers and plasticisers formulated to fill and cover hairline cracks and minor imperfections on ceilings and walls. They must be put on with a fluffy roller, which gives a slightly textured finish that's part of the cover-up. Odour-free, easy to use and soluble in water (making it easy to wash the roller out), they can cover in one coat, although two may work better on tricky surfaces. Bear in mind the plasticky additives could affect the porosity of the wall/ceiling and create a damp problem or make an existing one worse. Other base coats such as Polycell's Problem Wall Treatment are formulated to stabilise surfaces with flaky paint and loose plaster. For best results, make sure you remove as much of the loose stuff as possible first.
Products for more severely damaged surfaces are more closely related to plaster than paint, but for those nor wanting to employ a professional plasterer, there are supposedly "easy to use" options available that are generally non-toxic, with a low-VOC content. Not all are popular on the DIY chat boards, though, and they're not cheap, either ...
Skim coats
Skimming involves putting a thin coat of plaster on top of the existing plaster and is the best way to achieve a smooth finish on walls that are generally a bit battered or have large areas of damage or uneven surface, which would be difficult to finish well using a filler. Prior to skimming, a coat of a PVA (polyvinyl acetate) "bonding" is applied to stabilise the surface to provide something for the plaster to stick to. This is a job for a professional plasterer who will use all their experience to do the job far more quickly and more perfectly than you could ever hope to (unless you fancy enrolling in a plastering course yourself ). Despite the chat room complaints, a plasterer's fee normally represents good value for money, even when skimming.
Replastering
For walls with big holes, large cracks and lots of loose plaster, it is often necessary to get the wall, ceiling or whole room replastered. You could save money by taking off the old plaster yourself but, frankly, this is messy, hard work and you could end up taking off more than you should - thus making the job more complicated and expensive. Get a professional plasterer who knows about these things to take off the old as well as apply the new. Replastering requires two coats: a basic, coarser plaster followed by a smooth, thinner skim coat. For plasterboard walls with extensive damage it is better ask a builder and a plasterer to replace the plasterboard, which may be expensive but worth it.
One to avoid
These days most people want to get rid of any textured coatings, but believe it or not they are still being sold - not only as a decorative feature (mainly for ceilings) but as a cover-up solution for tatty surfaces. "Coarse" and "ripple" finishes are applied with a roller; others have to be applied using a wide filling knife. Frankly, these are best avoided as they generally look horrible - if surfaces are really bad, it is best to get them skimmed.
Before painting any surface, it should be free from dirt, grease, rust, dust and loose flakes of old paint. Some require primers and/or undercoats to provide a base suitable for painting ...
For new and unpainted wood
Paint over knots with "knotting", otherwise the sap in them will bleed through subsequent coats of paint. Apply wood primer followed by an undercoat (or an all-in-one). Use an oil-based undercoat with oil-based gloss or eggshell paint.
Unpainted metal Use a metal or all-purpose primer followed by gloss, eggshell or emulsion paint. Use an oil-based undercoat under oil-based paints.
Painted wood No undercoat required; just wash and sand it. An undercoat may help.
MDF Unpainted MDF needs sealing before painting. Use a specialist sealant such as Johnstone's MDF primer followed by an undercoat, before glossing or eggshelling.
Tiles Clean with sugar soap, then wipe over with white spirit to get rid of any dirt and soap scum. Use a specialist glass, tile or difficult-surface primer followed by paint, or Ronseal One-Coat tile paint (which needs no primer).
Melamine Treat like tiles: prime it using a tile and melamine primer or specialist product such as International Melamine primer or Dulux Difficult Surface primer. It can be difficult to achieve a good result on tiles or melamine.