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How to undercoat a wall

A man uses a paint roller to apply an undercoat to an interior wall.

Overview

When painting walls, it’s important to apply an undercoat, especially on a new wall. It smooths the surface, fills in any gaps, covers joins and provides a good, sound surface to paint on. For an older wall, choose a three-in-one formula to prime, seal and undercoat. This will help prevent stains from seeping through, as well as improve the new paint coverage.

Safety tip: Always wear the appropriate safety equipment (safety glasses and a dust mask, for example) and always follow the instructions for the product or equipment.

Steps

1Mask off with painter's tape

Start off by protecting the floor from drips and spills. Stick painter’s tape on the floor along the base of the wall, then cover the whole area with a drop sheet. Next, mask off all of the things you don’t want to paint. Run tape along the top of the skirting boards, around window and door trim, and around any switches and power points.
A pair of hands line interior skirting with painters’ tape.

2Prepare the wall

Fill any gaps and cracks with premixed filler and leave it to dry completely. Smooth over the filler using a hand sander with 120-grit abrasive paper. (Make sure you wear a dust mask and safety glasses when you’re doing this.) Brush down the wall with a soft-bristled broom to remove all dust.
A man uses Selleys Spakfilla to fill a hole in an interior wall.

3Cut in around trims and corners

You’ll need an angled brush to paint around power points, switches, door or window frames, around the edges of the wall, under the cornice and into the corners. This is called cutting in. Basically, it means painting anywhere a roller can’t reach neatly or easily. Brush paint evenly along the tape and feather it outwards towards the main part of the wall.
A man applies primer to an interior wall with an angled paintbrush.]

4Prepare your roller

A roller with an extension pole makes it easier to roll all the way from your corners to down to your skirting board. Make sure your roller fits the roller frame, which then fits the roller tray. For the roller cover, look for a nap of 10-15mm, which is good for the low sheen paints you’d typically use on walls. Stir the undercoat with a paint stirrer before pouring it into the roller tray.

Work from ceiling to floor, moving along about a metre, before reloading the roller and covering the next metre.

A man uses a paint roller to apply an undercoat to an interior wall.

5Finishing up

Leave the undercoat to dry thoroughly before applying your top coat. To do this, just repeat the masking, cutting in and rolling with your chosen top coat paint colour.
Green Bunnings hammer
Tip: Take off the painter’s tape as soon as you’ve finished. If you leave it until the paint is completely dry, it can lift the fresh paint. To remove the tape, hold the end and peel it away from the wall at a 45° angle to prevent smudging.

6Now you’re ready to paint the top coat

Follow our easy instructions on how to paint a room.

 

Photo Credit: Michelle Holden

Health & Safety

Asbestos, lead-based paints and copper chromium arsenic (CCA) treated timber are health hazards you need to look out for when renovating older homes. These substances can easily be disturbed when renovating and exposure to them can cause a range of life-threatening diseases and conditions including cancer. For information on the dangers of asbestos, lead-based paint and CCA treated timber and tips for dealing with these materials contact your local council's Environmental Health Officer or visit our Health & Safety page.

When following our advice in our D.I.Y. videos, make sure you use all equipment, including PPE, safely by following the manufacturer’s instructions. Check that the equipment is suitable for the task and that PPE fits properly. If you are unsure, hire an expert to do the job or talk to a Bunnings Team Member.