What’s the Reason to Sand After Priming?

by Charlie
What's the Reason to Sand After Priming

If you’re thinking of painting any surface, you’ll know that you need to sand it before you can prime it – but what about sanding it after priming? This might seem like an odd thing to do. You’ve already made the surface smooth, after all, and what’s the benefit of sanding it down a second time?

Sanding is a process many people hate, because it’s messy and time-consuming, and it results in dust getting all over you, your clothes, and the surrounding area. However, a lot of people say that you should sand after priming, so you might be wondering why this is recommended and whether it’s really necessary.

What’s the reason to sand after priming? Sanding after priming has a few advantages, particularly if you are painting wood. The grain of the wood may rise when you add the primer to it, because the primer is wet, and if this happens, you’ll get a slightly rough surface that will make your paint duller. Sanding also increases the amount of grip that the paint will have, making it stick better.

Do You Have to Sand After Priming?

You don’t have to sand after priming, but many professional contractors and experienced DIY enthusiasts will recommend that you do so, because it offers several benefits. This is particularly true when sanding wood, as mentioned above, but it holds true for almost any DIY project.

You might look at your primed surface and feel irritated by the idea of sanding it. After all, you have just sanded the whole area prior to putting the primer on it, and it’s perfectly smooth and looks ready for paint. If you do put paint on it without sanding, it should still stick well, without any major issues arising.

However, if you sand the surface, you’ll give it a much better chance of good adhesion. When you sand the primer, you make thousands of very tiny holes in its surface, and the paint will fill these holes, helping it to cling tightly and making it stick far better than if you skip the primer. You’ll have created a larger surface area for it to bond with.

Furthermore, sanding removes any tiny fibers left behind by the roller or brushes you used to apply the primer. While these aren’t generally large enough to show up, they can make a difference to the overall impression of your paintwork, and a lot of people prefer to remove them. Quickly sanding the surface down will leave it clean and perfect for paint.

Finally, it will get rid of any small bumps or lumps in the primer. Although you should try to avoid these, they may sometimes appear, especially if you have used a brush instead of a roller, or if your primer is a little old.

Sanding will smooth these bumps down so you have a flat surface to paint on. If you don’t sand, these imperfections may show up in your topcoat.

How Do You Sand After Priming?

It’s really important to take the right approach so you don’t strip off the primer you have just worked hard to apply. You only want to remove the top layer of the primer’s surface and make it smooth. You should therefore use a fine sandpaper, somewhere around 220 grit, so you are only taking a very small amount off the surface.

Work slowly and carefully, especially if you are using a machine to sand with, and do not press too hard. If you’re going to use a power sander, consider a higher grit sandpaper, perhaps around 320, so that it doesn’t take off too much at once. Remember that high grit sandpapers are ideal for creating a key that paint can stick to.

Always remember to wear a mask and eye protection when using sandpaper. Sand the whole surface as evenly as you can, working from one edge to the other so you know you haven’t missed any parts.

When you’ve finished sanding, wipe the surface down with a damp cloth to remove all the dust, and then allow it to completely dry before you try painting it. You should then be able to achieve a beautifully smooth coat of paint.

How Soon Should You Sand After Priming?

This depends heavily on the kind of primer you have used. The primer needs to be completely dry before you start sanding it, so check the tin to find its drying time. Most latex or water-based primers will dry in around 5 or 6 hours, while oil-based primers usually take about 12 to 15 hours.

Bear in mind that cold and dampness will slow down the drying process, so if you’re working in a non-ideal environment, you need to add to the expected drying time. Only when the primer is completely dry is it ready to sand. If you try to sand wet primer, it will stick to your sandpaper, tearing off the surface, and you’ll make a mess of the whole project.

Before sanding, firmly press your finger against the surface of the primer, and see if any residue comes away on your skin. If it does, the primer isn’t ready to sand yet, and you’ll need to wait longer.

Do You Only Need to Sand Primer on Wood?

The wood grain often raises slightly when the primer is applied to it, because the moisture in the primer causes the wood to swell. This gives you a rougher surface, even if you can’t see it, and it’s why sanding primed wood is particularly important. However, sanding primer on any surface will give you a better finish, and should not be neglected.

Whether you are painting metal, wood, plaster, plastic, or any other surface, sanding the primer should improve the adhesion of your paint.

Final Thoughts

Sanding after you have primed is a frustrating task, but it will give you a much better finish, and it’s worth doing. It removes imperfections in the primer layer, improves the key, and makes your finished paintwork look much more professional. Use some fine sandpaper and take the time to sand down the surface before you start painting.

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