PREVIOUSLY PAINTED SURFACES
Test the paint in several areas by cutting a small cross with a sharp knife and press 10 cm or so of adhesive tape firmly across the middle of the cut. Rip the tape away quickly - if any pieces of paint come with it, you'll need to strip the loose paint.
If your paintwork has failed the adhesion test or it's heavily layered it should be stripped back to a firm base before repainting.
The most common methods of stripping are a heat gun or chemical stripper, although for small areas of badly damaged paint a manual or drill-mounted wire brush or dry scraper may be adequate
If the paintwork on timber is in good condition, simply sand thoroughly with fine sand paper to establish a key for the new paint. Any small areas of peeling or cracking may be sanded back, the edges feathered so there is no visible defect and spot-primed if bare timber is showing.
Holes or defects should be filled with exterior filler and spot-primed.
Painted brick or masonry should be washed with a high pressure cleaner or water and a stiff bristled brush.
Generally speaking, damaged paintwork requires removal of the paint as outlined under the Stripping heading. However, there is no point in removing the effect without treating the cause. In the examples below-we'll look at some common symptoms and their causes.
Blistering, Flaking and Peeling
When this occurs on wooden surfaces the cause is usually moisture trapped beneath the paint. It occurs most frequently on the north and west sides, as these areas receive the most heat from sunlight which, over time, weakens the paint. This is most common where dark colours have been applied over many layers of old paint.
The first thing to do is remove the source of the moisture penetrating the substrate.
Around windows and doors, look for cracks and seal them with caulking. In walls the problem may be condensation, so install additional vents.
Strip and prepare as outlined above.
Cracking and Alligatoring
This may be caused by applying too thick a top coat over a soft of incompletely dried primer, or multiple coats that are too thick to allow the earlier coats to dry completely. Strip as much as practicable, sand smooth and prepare as outlined above.
Flaking and Chalking On Masonry
Sometimes paint on masonry may flake off and leave a chalky or powdery residue. On new masonry this is probably caused by using a paint that is not formulated for masonry (not alkali resistant), poor surface preparation or moisture. Any masonry surface must cure for 4 to 8 weeks prior to painting or undercoating.
The first thing to do is to check that there are no abnormal moisture sources inside your house, especially if this is occurring close to bathrooms or plumbed areas. Locate and correct any problems before proceeding with the surface preparation.
If the surface is only slightly chalky, a good scrub and hosing down is all it will need. If the paint is very loose and peeling you may need to remove it entirely with a wire brush. There must not be any loose paint left on the surface.
Chalking On Timber
Old oil based enamel paints break down over time in our high UV sunlight to create a chalky or powdery deposit on the surface. This is particularly noticeable on dark colours. This should be scrubbed off and the adhesion of the old paint tested before repainting.
Moss and Lichen
Moss and lichen must be removed and the area thoroughly cleaned before painting. Use a stiff brush or broom to remove loose growth, then apply a de-mossing chemical such Bordeaux Mixture which is available from nurseries. Apply it to all the affected surfaces and leave for 3-6 days. Then sweep the treated areas with a stiff brush or broom and rinse with copious amounts of clean water. Allow to dry. Treat the area with a bleach solution as you would for mould and allow to dry out completely before applying paint.
A FINAL CHECK
Before painting, give the area a last clean and check. Try to remove as much dust as you can and thoroughly wipe over the walls and woodwork with a damp cloth.