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Using old cans of primer is a good way to save money and protect the planet from unnecessary waste. But is it safe to use exterior primer on interior surfaces?
Never use shellac or oil-based primer inside as it will fill your indoor space with toxic fumes. However, you can save money by using a can of latex-based primer leftover from an exterior painting project, but it’s overkill. You don’t need the stain-blocking qualities of exterior primer for fresh drywall.
This article is going to take a closer look at the 3 different types of primer available so that you can make an informed decision on which to purchase.
What’s the difference between interior and exterior primer?
Primers are usually made for interior and exterior surfaces and more metals. Interior primer is designed to seal the surface, provide a lot of adhesion for the paint, and create a smooth, uniform surface for the paint to go onto.
Exterior primers are available in specific types for metal, wood, and masonry, and are usually used to prevent the paint from cracking and to prevent the growth of mold and mildew. They are also used to protect masonry from efflorescence and from alkalinity.
3 Types of primers
There are three different types of primer that you can use, which are latex, oil-based, and pigmented shellac. Each type of primer has different properties that make them suitable for use on different surfaces and in different situations.
Oil-based primers are extremely versatile, making them suitable for use on many different surfaces. This includes wood, steel (and other types of metal), and surfaces that already have already been painted, including internal and external walls. Oil-based primers are also suitable to use with both latex and oil paint.
Oil-based primers are a particularly good choice for bare or unfinished wood as it seals the surface. Sealing means that the paint will cover the surface better and provide a better finish. They also prevent the tannins that are released in some types of wood from seeping through the paint. They also prevent the paint from blistering or peeling.
This type of primer is also known as a “stain killer” as it stops stains from being visible through the paint. It also prevents stains such as ink, water, or nicotine from showing on the walls.
This stain-killing feature makes oil-based primer an excellent choice for areas of your home that might be handled frequently by children, such as doors, walls, and cabinets, as it will seal in old stains and prevent new ones.
An oil-based primer is also a suitable choice if the surface you are painting is going to be exposed to the elements or to extremes of temperature as it can withstand temperature changes.
However, there are some drawbacks to using oil-based primers (and oil-based paints) as they do release a lot of volatile organic compounds (VOCs), which can be harmful to you if you are exposed to them over a long period of time or in a high concentration.
Oil-based primers also take a long time to dry (around 24 hours), and you will usually need to use strong solvents and paint thinners to clean your brushes or rollers.
You should not use an oil-based primer on masonry. It is much more suited to wood (especially unfinished wood), patched walls, raw drywall, and other stained surfaces. When you are using an oil-based primer, you should always plan ahead to allow for the 24 hours that the primer will need to dry.
If you need to thin the primer, then you should add a little mineral spirit and stir it in before applying the primer. To make sure that you get a smooth finish, you should use a natural bristle paintbrush.
Latex-based primers are quick-drying, taking only 3 to 4 hours to dry before you can begin painting. They are a good choice for plaster, drywall, wood, masonry, and metal. However, it is recommended that you do a test patch first to ensure its suitability for the surface you are painting.
Latex-based primers are water-based and are not as brittle as oil-based primers. This means that there is less risk of them cracking or peeling. This makes them a good choice for preparing unfinished drywall for painting, priming softwood, and also for use on concrete and galvanized metal.
Most latex-based primers contain little to no VOCs, which makes them suitable for use indoors and a particularly good choice for unfinished drywall. However, even though it does not contain as many harmful substances, you should still make sure that you allow plenty of ventilation into the room, using a fan if necessary.
However, the drawback of using a latex-based primer is that does allow stains to show through it. This is because it is not as thick or durable as the other types of primer. Despite this, as it is water-based, it does mean that it is easy to clean up if necessary.
If you are using a latex-based primer, you should leave enough time to do a test patch first. This is especially true if you are painting wood as you will need to see if the primer will cause the grain to rise. Although patch testing will add time to your project, it is worth it to make sure that it doesn’t damage the surface.
Shellac is something that has been used for a long time for sealing wood and many other materials. This type of primer is a good choice for interior paintwork, and it has excellent stain-blocking properties. It can block smoke, water, and rust stains, including severe ones. It also prevents tannins in wood from seeping into the fresh paint.
Shellac primers are quick-drying and highly adhesive. They are suitable for use on metal, wood, plaster, and plastic. This type of primer is also suitable to use with both latex and oil-based paint. However, despite its stain-blocking properties and smooth coverage, shellac primer does contain quite a higher number of VOCs than latex primers.
You should always ensure that you have plenty of ventilation in the area you are working. It’s a good idea to open all of the windows and use a fan as well. Most people use natural bristle brushes, but you can use either natural bristle or synthetic paintbrushes with shellac primer. To thin shellac primers and to clean your paintbrushes you will need to use denatured alcohol.
Despite the few drawbacks, shellac primer is an excellent choice for interior work as it has a lot of good qualities. It is excellent for sealing surfaces and blocking stains, and that is why it has been highly regarded for hundreds of years.