Leo Martínez
Painting new paint on top of old paint is a simple process, but there are a few things to keep in mind. The chemistry of the paints is important, as oil-based paint will not adhere to latex paint without a primer. If you are painting a lighter colour over a darker one, you will likely need multiple coats of paint or a primer. Filling holes and cracks and sanding the surface will also help to ensure a smooth and even finish.
| Characteristics | Values |
|---|---|
| Surface preparation | The surface should be sound, clean, smooth, dry, and free of loose or flaking paint. |
| Paint adhesion test | Cut a small "X" through the existing paint, apply sticky tape, and rip it off. If the paint comes off, it is unsound and should be removed. |
| Lead paint | If lead paint is suspected, hire a professional to test and remove it. |
| Primer | A primer may be necessary, especially when changing paint types (e.g., from oil-based to water-based) or covering imperfections. It helps prevent the old color from bleeding through and provides a smooth base layer. |
| Number of coats | Multiple coats may be required when covering a dark color with a lighter one or when using opposite shades. |
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What You'll Learn
When to use primer
Painting over old paint requires a sound surface for the new paint to adhere to. If the old paint is not adhering properly, it should be removed by scraping, sanding, or with chemical strippers. If there is lead paint present, a professional should be consulted.
Primers are used to ensure paint adheres to a surface and to seal that surface. They are especially useful when dealing with bare or unusual surfaces, such as bare wood or sheetrock, or when painting over old wallpaper paste. If you are painting over oil-based paint with a water-based paint, a bonding primer is necessary. Latex paint, for example, will not bind to oil without a primer. Premium acrylic paints can be applied directly over oil-based coatings if the surface is properly prepared by washing and sanding.
Primers are also useful when making drastic colour changes, as they can be tinted towards the colour of the new paint, helping to cover the old colour in one coat. This can be especially useful when painting over a bright red wall with a lighter colour. Tinted primers can also be used to cover tough stains.
In some cases, a second coat of paint may be sufficient to cover old paint without the need for a primer. However, this may not be as effective as using a primer, especially when dealing with porous surfaces such as drywall, which can absorb new paint unevenly, resulting in flashing and lap lines.
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Paint adhesion test
When painting over existing paint, it is crucial to ensure that the new paint will properly adhere to the old paint. This can be done through a Paint Adhesion Test.
One common method for testing paint adhesion is the cross-hatch or cross-cut test. This test involves cutting small squares or hatches into the paint with a blade, which reduces the lateral bonding. The blade should only penetrate the previous paint layer, and not the underlying substrate. This can be done freehand or with a specialised kit, such as the Paint Adhesion Test (PAT) Kit by GARDCO, which includes a blade holder/handle, hex wrench, flaking brush, handheld magnifier, tape, instruction guide, and a carrying case. The blade spacing and the number of cuts will depend on the thickness of the paint film. After making the cuts, pressure-sensitive tape is applied and removed. If the paint comes off with the tape, it indicates poor adhesion.
Another method is the pull-off adhesion test, where a loading fixture, or dolly, is affixed to the coating with an adhesive, and then pulled off with a portable pull-off adhesion tester. The force required to pull off the dolly, or the force it withstood, indicates the tensile strength in pounds per square inch (psi) or mega Pascals (MPa). This test maximises tensile stress compared to other methods, such as the scrape test or knife adhesion test, which use a knife or stylus to pick or push at the coating.
A simple test for adhesion that can be done at home is to cut a small "X" through the existing paint with a sharp blade, then press and rip off tape. If the paint comes off, it is unsound and should be removed. This test should be repeated in multiple areas to adequately test the surface.
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Preparing the surface
Inspect the Condition of the Wall
Begin by evaluating the overall condition of the wall. If the wall is in good condition, with no significant damage or imperfections, you may not need to apply a primer. However, if there are holes, cracks, or other flaws, they will need to be addressed before painting.
Determine the Type of Existing Paint
Identify the type of paint currently on the wall. Is it oil-based, latex, or another variety? This information is essential because certain types of paint are incompatible. For instance, oil-based paint will not adhere properly to latex paint.
Perform the Paint Adhesion Test
To ensure that the new paint will adhere well to the old paint, conduct the Paint Adhesion Test. Cut a small "X" through the existing paint with a sharp blade, press sticky tape firmly over it, then pull off the tape. If the paint comes off, it indicates poor adhesion, and you should remove the unsound paint by scraping or sanding before proceeding.
Choose the Right Primer
If the wall has imperfections or if you're switching from a water-based paint to an oil-based one (or vice versa), it's advisable to use a primer. Select a bonding primer suitable for your specific situation, such as a primer that can cover oily spots near a stove to enable the application of latex paint.
Apply the Primer
After selecting the appropriate primer, apply it evenly to the wall. Depending on the product and the manufacturer's instructions, you may need to apply two coats of primer to ensure a consistent base layer for the new paint to adhere to properly.
Sand and Clean the Surface
Before applying the new paint, lightly sand the surface to create a smooth base. Use a fine-grit sandpaper and work gently to avoid damaging the wall. After sanding, remove any dust or residue with a damp sponge or cloth, ensuring the surface is clean and dry.
By following these steps, you will effectively prepare the surface, ensuring that your new paint job adheres properly and results in a high-quality, long-lasting finish.
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Types of primer
Before painting over existing paint, it is important to ensure that the surface is sound for paint adhesion. This can be done by performing a paint adhesion test. If the paint is unsound, it should be removed by scraping, sanding, or with chemical strippers. If there is lead paint present, it is recommended to have a professional test and remove it.
There are several types of primers available, each with its own advantages and suitability for specific surfaces and conditions:
Oil-Based Primers
Oil-based primers are versatile and can be used with both oil-based and latex paints. They are suitable for a wide range of surfaces, including wood (painted or unpainted), steel, metals, and interior or exterior walls. These primers are effective in preventing stains from showing through new coats of paint and are ideal for blocking stains from ink, nicotine, and water. They are also suitable for sealing porous surfaces, such as softwood, concrete, and brick. However, oil-based primers have longer drying times and release higher amounts of volatile organic compounds (VOCs). They require careful disposal and the use of harsh thinners for cleaning brushes and applicators.
Latex Primers
Latex primers are water-based and are ideal for prepping unfinished drywall for painting. They are more flexible, fast-drying, and less brittle compared to oil-based primers, making them less prone to peeling and cracking. Latex primers are suitable for softwood, brick, concrete, and galvanized metals. They are also recommended for rusty surfaces, as they provide corrosion resistance.
Pigmented Shellac Primer
Pigmented shellac primer is effective in blocking smoke and soot damage, as well as eliminating odours from animal urine.
Bonding Primers
Bonding primers have strong adhesive properties and can adhere to various surfaces such as glass, tile, Formica, and previously painted surfaces. However, they are recommended for interior use only, as they tend to crack when exposed to outdoor elements.
Tinted Primers
Tinted primers improve the final outcome of your painting project by working with the finish coat colour to enhance colour accents. They are particularly useful when making drastic colour changes or patching, as they help the topcoat achieve better coverage.
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How many coats?
When painting, the number of coats you will need depends on a variety of factors, such as the type of surface, the condition of the walls, the quality of the paint, and the colour choice.
If you are painting a previously painted wall, the number of coats can depend on whether you are painting over it with a similar or darker colour. When covering a light-coloured wall with a darker colour, you may only need one coat with a few added touch-ups. However, if you are painting a darker wall with a lighter colour, a minimum of two coats is required for complete coverage. If the colour change is drastic, such as going from black to white, you might need three coats.
The condition of your walls is also important to consider. If your walls are in good condition, one or two coats of paint may be sufficient. However, if your walls are rough or damaged, additional coats may be needed to cover imperfections. Applying a primer can help fill in imperfections and ensure a more uniform appearance.
The quality of the paint also matters. Higher-quality paints generally require fewer coats than lower-quality paints. A paint with good quality ingredients and pigments will provide better coverage and durability.
To achieve a smooth and durable finish, it is recommended to apply at least two coats of paint. Thin layers of paint are best, allowing each coat to dry fully before adding the next.
In summary, the number of coats needed depends on the specific circumstances of your painting project. Considering factors such as surface type, wall condition, paint quality, and colour choice will help you determine the appropriate number of coats required.
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Frequently asked questions
Using a primer is not always necessary, but it is recommended if you are painting over a darker colour with a lighter one, or if the paints have different chemical compositions (e.g. latex and oil-based paints).
One coat of primer is usually sufficient, but you may need two coats if you are covering a very dark colour or filling in holes and cracks.
You can use a clear or white primer, or have it tinted to match your paint colour. Tinting the primer a neutral grey can also help reduce the number of topcoats needed.
Start by cleaning the surface with a damp cloth to remove any dust, dirt, or grease. If there are any holes or cracks, fill them with a suitable filler and sand the area smooth. Then, apply your primer and paint.
If the chalk paint has been sealed with an oil-based wax, you will need to remove the wax first with rubbing alcohol or odourless mineral spirits. You can also test for wax by running your fingernail across the surface or trying to sand the area.
