As you read the back of your paint can, the company might recommend you use a certain primer before applying the paint. Many homeoweners choose to skip this step and begin painting right away. The paint you chose may even claim to have paint and primer in one, giving you the conception that the priming step is acceptable to skip. So what's the answer? Do you really need to primer separately before painting?
The answer is... it depends. You don't always need to primer before painting. It all depends on what surface/material you are wanting to paint over. There are three main purposes for primer: sealing, gripping, and stain blocking.
Sealing primers are usually used for sealing porous and raw materials, prepping them for paint application. Examples are fresh drywall, bare wood, bare metal, concrete, new stucco, etc. It is important to prep and prime these kinds of surfaces before painting.
Gripping primers are used when you are wanting to paint over hard to stick surfaces, such as oil based glossy surfaces. Just like when you get paint on glass, it can be easily scraped off with a razor because the surface is smooth and glossy. But if you were to primer the glass with gripping primer before paint, it would be much harder to scrape the paint off the glass. On the other hand, if you were to get paint on a more porous surface, such as brick, the paint will adhere and soak into the surface much better. Gripping primers are made to adhere to any surface really well, so don't get them on your hands! In the painting industry, gripping primers are used often when painting over oil-based paints/finishes. Water-based paints do not adhere well to oil-based finishes. Therefore, apply a good gripping primer over the oil-based paint will ensure that the new water-based paint will adhere well to the surface.
Stain blocking primers are pretty self explanatory. They are used to cover strong stains that may bleed through if you were to apply paint over the surface. Such stains include: water stains, wood stains, dirt/grime, ink, etc. Be sure to wear gloves and a respirator while handling these primers. Stain blocking primers usually come in oil and shalac based solvents in which the fumes released are very strong.
So, when DON'T you need primer? There are a few exceptions where one does not tecnichally need to primer before paint. When you are painting over a previously painted surface that is clean and more matte in finish, you do not need to primer beforehand. For example, if you are wanting to freshen up your bedroom walls and ceiling with a fresh coat of paint, you may not need to primer beforehand. If the ceiling and walls are clean and not too glossy in sheen, then you can get away with applying two coats of paint and skipping the primer step. Keep in mind, it is always good to lightly sand and clean surfaces before paint.
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