As a Professional Decorator, it’s fair to say I have done a lot of prep over the years. I thought I’d sit down and write a quick blog on how to prep walls before painting.
I’ll quickly take you through the materials and tools you need. I’ll then briefly list all the steps, followed by product recommendations. I’ll finish off by talking about prepping walls in different scenarios, so you can adapt your method for different jobs.
How to Prep Walls Before Painting
Supplies
Interior filler – Click here to see online prices.
Sandpaper – Click here to see online prices.
Tools
Filling knife – Click here to see online prices.
Dust brush – Click here to see online prices.
Paint roller – Click here to see online prices.
Paint brush – Click here to see online prices.
Steps
Prime bare plaster
Skip this step if you’re prepping previously painted walls. But if you’re working on bare plaster, then the first thing you should do is apply ‘a mist coat’. There are different products you can use; I always recommend following the instructions on the paint you’re using as a topcoat.
Sand the walls
Previously painted, or newly mist coated walls should be sanded smooth. This helps you achieve a better finish.
Fill imperfections
Use a good quality interior filler to repair imperfections on your walls. Allow the filler to set, and then sand smooth.
Get rid of dust
Paint will not stick to dust. You should remove it with a dust brush or something similar.
Prime the filler
Large patches of powdery filler will need to be primed before you can paint. You can usually do this with a diluted coat of emulsion. Failing to prime will result in flaking paint.
Should You Sand Previously Painted Walls Before Painting?
In short, yes, you should sand walls before painting. This is for a couple of reasons; Firstly, it helps smooth out imperfections that would otherwise show through your finished paint work.
The other reason is adhesion; You need your new paint to stick to the old paint. This isn’t always an issue, but it can be if you’re painting over eggshell, silk, or contract matt.
If your walls are in good nick, you might just need to surface fill any blemishes, lightly abrade the whole wall, then just give it a quick dust off, and you’re ready to paint. Axus sandpaper is probably the best, but any will do.
How to Prep Newly Plastered Walls
Newly plastered walls are a little bit different. A lot of people make the mistake of sanding new plaster before painting it. However, this can coat the wall with a thin layer of dust, which causes adhesion issues and results in paint peeling further down the line.
Instead, inspect the walls and scrape off any plaster snots (lumps of plaster that have fallen off the trowel onto the wall). Then apply a ‘mist coat’ to prime the walls. The product you use as a mist coat will be named on the instructions of the product you are using as a topcoat. Normally your mist coat is a diluted coat of the same product as your topcoat, but some brands (like Farrow and Ball and Tikkurila) specify a different primer, so always check.
Once mist coated, allow to dry, do any filling and sanding as required, then crack on and paint your walls.
How to Prep Walls That Are in a Bad State
Depending on how bad your walls are, you may need to surface fill and sand your entire wall. If it still isn’t smooth enough to paint, then installing lining paper may be an option. Or you could skim the walls yourself using roll-on filler, which is surprisingly easy to do with the right materials.
Surface Filling
There are plenty of fillers you can use as part of your prep before painting walls. Powder filler (the type you mix yourself) is generally easier to sand smooth after you’ve applied it. Go for an interior filler too, as some of the multipurpose products contain cement which can flash through your paint.
My suggestion would be Gyprock Easifill, which is easy to use, readily available, and reasonably priced. Click here to see online prices.
Simply add some water to a tub or bucket, then add the filler, and mix until you reach a smooth consistency. Then apply a thin coat over all the imperfections on your wall using a filling knife.
You should open any cracks with the edge of a scraper before filling so the filler can penetrate.
Once dry, simply sand smooth, dust off, and paint your wall. Your first coat of paint over filler should be diluted to aid adhesion.
Occasionally you may notice the odd imperfection after you’ve applied your first coat of paint. No need to panic, just fill any bits, sand, spot prime with paint, then apply your topcoat.
Skimming Walls with Roll-On Filler
This way of prepping your walls before painting is going to cost you a little more, and it is more physical. But it’s easy enough to achieve a good finish and it’s a lot cheaper than hiring a plasterer. I’ve been using a new product called Dalapro Roll Nova. You just apply it with a paint roller and then smooth it off with a filling blade. Once dry you just give it a quick sand, dust it off, then you’re ready to paint.
Lining the Walls Before Painting
You will still need to fill most of the imperfections on a wall before lining, so it isn’t a quick fix. However, lining your walls can be a great way to combat reoccurring cracks and movement on your wall, so your paint finish will last longer.
I like using 1400 grade lining paper, which can be bought online by clicking here. You can apply it horizontally or vertically, and you’ll find it easier to use than normal wallpaper. Click here for insight on the best wallpapering tools.
My favourite paste is Beeline, but any ready mixed paste is generally fine. Avoid flake (paste you mix yourself) because it isn’t as strong, and your lining paper can bubble when you’re painting it.
How to Prep Walls That Were Previously Wallpapered Before Painting
You should take a few additional steps when prepping a previously wallpapered wall before painting, simply because your paint can react with leftover paste.
Some people vigorously wash the walls down with a cleaning agent called Zinsser DIF. This is great if you can get all the paste off the wall, but there is an easier way.
I allow everything to dry properly, sand the wall smooth, then apply a coat of Zinsser Gardz. This acts as a barrier coat, stopping your paint from touching your paste and reacting with it. Zinsser Gardz is thin, easy to apply, clear, and goes for miles. Click here to see online prices.
FAQs
What to wash walls with before painting?
Only wash walls down as part of your prep if they are dirty. If they do need washing, go for either sugar soap or Pretty Boys. Just dilute the agent with warm water and wipe the walls down.
What happens if you paint walls without sanding?
Probably nothing. Sanding aids adhesion and ensures a smoother finish. However, as a Painter and Decorator, before I bought dust free sanding equipment, I’d only sanded parts of the walls that I though needed smoothing. I never had an issue.
What happens if you use old paint on walls?
Old paint can smell a little bit. This is because bacteria in the paint eat the starch and the biproduct stinks. If you use old paint on your walls, then your house will smell for a couple of weeks, but it will dissipate.
Do I need primer if my wall already has old paint?
Most paints do not require a primer when you’re applying them to previously coated walls. There is the odd exception (like Farrow and Ball). Or you might need a primer if painting over oil-based paint. The best advice I can give you is to read the manufacturer’s instructions.
Can you paint over flaking paint?
Flaking paint on walls or ceilings can be a nightmare, and it’s an issue that can occur for a few reasons. If you were to just paint over flaking paint, then no matter how good your new paint is, your previous coating may continue to fail and ruin your newly painted walls. You should remove as much of the flaking paint you can and then stabilise with Zinsser Peel Stop as part of your prep.
Final Thoughts
Prepping walls before you paint isn’t complicated. I’m lucky in that I have dust free sanding equipment which makes things cleaner, quicker, and safer. Without it, you will make a mess unless you take steps to cut down on dust. I’d also recommend wearing a mask.
Other than that, it’s quite straightforward. Happy decorating.
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