Plasterboard is one of the most widely used materials in modern construction for creating internal walls and ceilings. Once the boards are fixed in place, the next step is preparing the surface for decoration. Unlike traditional wet plaster, plasterboard comes with visible joints between sheets, fixing holes from screws, and external corners that need proper treatment before any paint or wallpaper can be applied. This article walks through the practical steps for filling plasterboard joints and preparing the surface so that your finished decoration looks seamless and professional.
The key to a flawless finish lies in the preparation. If you simply paint directly over untreated joints, the gaps and screw heads will remain visible through the paint, ruining the overall appearance. The process involves using joint tape, specialist filler compounds, and the right tools to achieve a smooth flat surface. You can learn more about the different painting techniques for different surfaces to understand how preparation varies between materials.
Tools and Materials You Will Need
Before starting any plasterboard filling work, gather the necessary tools and materials. Using the correct equipment makes the job significantly easier and produces better results. Here is a checklist of everything you are likely to need:
- Jointing compound or easi-fill powder filler
- Plasterboard joint tape (paper tape or self-adhesive mesh tape)
- Jointing knives in three sizes: 150 mm, 200 mm, and 300 mm
- Steel angle beads for external corners
- Mixing bucket and clean water
- Medium grit sandpaper (around 100 grit) and fine grit sandpaper (120 grit or higher)
- Sandpaper block or pole sander for large areas
- Dust mask and safety goggles
- Filling knife for screw head indents
- Paintbrush and roller for applying mist coat
- Matt emulsion paint for the mist coat
- PVA glue if using the PVA sealing method for wallpaper
Jointing knives are especially important because using the correct width at each stage helps you feather the filler out smoothly into the surrounding board surface. Narrower knives are used for the first coat and wider knives for subsequent coats, allowing you to blend each layer into a larger area. The mist coat technique for new plaster surfaces is an essential method that works equally well on sealed plasterboard.
Preparing Plasterboard Joints with Tape and Filler
The tapered edges along the long sides of plasterboard sheets are designed to accept jointing compound, creating a recess that can be filled flush with the board surface. The short ends of boards are not tapered and require a slightly different approach. Here is how to handle both situations correctly.
Tapered edge joints (long edges): Apply a thin layer of jointing compound along the entire length of the joint using the 150 mm jointing knife. While the compound is still wet, press a length of paper joint tape into the filler, centring it over the joint line. Run the knife firmly along the tape to embed it and squeeze out any excess compound from beneath. The tape must be fully bonded with no air pockets. Apply a second thin coat of filler over the tape, feathering the edges outwards by about 100 mm on each side.
Butt joints (non-tapered ends): These joints sit flush with the board surface rather than in a recess, so they require extra care. Apply the jointing compound and embed the tape in the same way as tapered joints, but feather the filler out much wider to avoid creating a visible ridge. Aim for a spread of 200 to 300 mm on each side of the joint. This wider feathering helps disguise the slight bump that naturally occurs at butt joints.
Once the first coat is dry, apply a second coat using the wider 200 mm knife, feathering even further. After drying, a third coat with the 300 mm knife may be necessary for butt joints. Sand lightly between coats with medium grit paper. Understanding how primer paints work on different substrates will help you choose the right sealing approach once the filler work is complete.
| Joint Type | Knife Size for Coat 1 | Knife Size for Coat 2 | Knife Size for Coat 3 | Feather Width Each Side |
|---|---|---|---|---|
| Tapered edge (long) | 150 mm | 200 mm | Not required | 100 mm |
| Butt joint (short end) | 150 mm | 200 mm | 300 mm | 200 to 300 mm |
| Internal corner | 150 mm | 200 mm | Not required | Feather both sides |
| External corner | 150 mm | 200 mm | 300 mm | Feather both sides |
Filling Screw Heads and Fixing Holes
Every screw used to fix the plasterboard to the underlying frame leaves a small indent that must be filled individually. Missing this step is one of the most common DIY mistakes and results in visible circular marks after painting. Follow this simple process for each fixing point:
- Check that each screw head is driven just below the surface of the board without breaking the paper facing. Screws that stand proud will need to be driven deeper.
- Using the 150 mm jointing knife, apply a small amount of filler directly over each screw head, pressing it firmly into the indent.
- Draw the knife across the filled indent in one smooth motion to remove excess filler and leave the surface flush with the surrounding board.
- Allow the filler to dry completely. This usually takes between one and two hours depending on the room temperature and humidity.
- Once dry, lightly sand the filled area with fine grit sandpaper until it feels completely smooth to the touch.
- Apply a second thin coat if the filler has shrunk below the surface level, then sand again.
Take your time with this stage. Rushing through screw filling often leads to visible spots in the final decoration that require sanding back and repainting. For more detailed guidance on achieving a smooth wall surface, see the section on painting over a freshly plastered surface, which covers similar preparation principles.
Dealing with Internal and External Corners
Corners require different techniques depending on whether they are internal or external. Getting corners right is essential because they are highly visible and any imperfections catch the light, making them obvious even from across the room.
Internal corners: Apply jointing compound into the corner with the 150 mm knife, then fold a length of paper tape along its centre crease and press it into the corner. Run the knife down each side of the tape to embed it fully. Allow the first coat to dry, then apply a second coat using the 200 mm knife, feathering out onto both adjoining walls. The finished internal corner should have a clean, crisp line with no visible ridge on either wall surface.
External corners: These require additional protection because they are vulnerable to knocks and impact damage. Fit a steel angle bead to the external corner, securing it with screws or a staple gun. Mix jointing compound and apply the first coat using the 150 mm knife on both sides of the bead, working right up to the 90 degree corner apex. Remove any excess compound and feather the edges out into the board surface. Allow to dry, then apply a second coat with the 200 mm knife and a third coat with the 300 mm knife. Each successive coat should be feathered wider than the last. The wallpaper preparation and sizing guide explains how to seal and prepare corners properly if you plan to apply wallcoverings instead of paint.
Once the filler has fully dried, sand all corner surfaces carefully with fine grit paper. Pay special attention to the edges of the filler to ensure there are no hard ridges that would cast shadows when painted.
Sealing and Painting Over Plasterboard
Plasterboard is highly porous, even more so than traditional plaster. If you apply finishing paint directly to untreated plasterboard, the moisture is absorbed unevenly, causing patches and poor adhesion. The surface must be sealed first using a mist coat.
A mist coat is simply matt emulsion paint that has been thinned with water. The standard ratio is approximately 60 percent paint to 40 percent water, though some brands recommend a 70/30 mix. The thinned paint soaks into the porous surface rather than sitting on top, creating a stable base for subsequent coats. Apply the mist coat evenly across all plasterboard surfaces using a roller, working in manageable sections. Allow the first coat to dry fully before applying a second mist coat.
Once both mist coats are dry, you can proceed with your chosen finishing paint. Two top coats of quality emulsion or vinyl matt paint typically provide excellent coverage and durability. For bathrooms and kitchens where moisture resistance is important, use a suitable kitchen and bathroom paint after sealing. Knowledge about repairing damaged plaster walls is useful if you discover any underlying issues while preparing the surface.
Summary of the sealing process:
- Mix matt emulsion paint with water (60/40 or 70/30 ratio)
- Apply the mist coat evenly with a roller to all plasterboard surfaces
- Allow the first coat to dry fully before applying a second coat
- Sand lightly with fine grit paper between mist coats if the surface feels rough
- Apply two finishing coats of your chosen emulsion paint
- Allow adequate drying time between all coats
Wallpapering Directly onto Plasterboard
If your chosen finish is wallpaper rather than paint, the sealing requirement is slightly different. The same mist coat method works perfectly as a sealant before wallpapering. Alternatively, you can use a sizing solution, which is specifically formulated for preparing walls for wallpaper. Sizing prevents the paste from being absorbed too quickly into the porous board, which could cause the paper to lift or blister during drying.
Another option is to use diluted PVA glue as a sealer. Mix equal parts PVA and water (a 50/50 ratio) and brush the solution over all plasterboard surfaces. Allow the first coat to dry completely, then apply a second coat. Once the second coat is dry, you can hang wallpaper exactly as you would on a conventional plastered wall. The PVA method creates a slightly glossy film that prevents moisture from the wallpaper paste soaking into the board while still providing adequate adhesion.
Whichever sealing method you choose, ensure all joints, screw heads, and corners have been properly filled and sanded before you begin. Imperfections that are barely noticeable on a painted wall become glaringly obvious under wallpaper because the paper follows the surface contours exactly. A flat, smooth substrate is the foundation of professional-looking wallpaper installation.
Successfully preparing plasterboard for decoration comes down to patience and attention to detail. Filling joints in stages with progressively wider knives, sanding between coats, and sealing the surface correctly will reward you with a finish that looks every bit as good as traditional wet plaster at a fraction of the cost. The techniques described here apply whether you are working on a new partition wall, a ceiling, or a dry-lined external wall, making them essential knowledge for any DIY enthusiast tackling interior finishing work.