How to Use Joint Compound for Plasterboard

If you’re a plasterer or regularly do larger home projects, you’ll likely be used to working with plasterboard. These dried gypsum sheets make it easy to build new walls or ceilings, although doing a professional job still requires a lot of knowledge.

Arguably one of the most important pieces of kit for finishing a plasterboard wall is joint compound. Whether you’re a beginner or just need a refresh, this blog will go through why joint compound is so important and the simple process for applying it to your plasterboard.

The Importance of Joint Compound for Plasterboard

Joint compound – also known as drywall mud due to its consistency – is a sticky, spreadable paste that sets to create a smooth finish on plasterboard and drywall installations. It can be purchased as a powder which is then mixed with water or in ready-mixed tubs.

Join compound is primarily made of gypsum, the same material used to make plasterboards, combined with various other agents. Once applied to plasterboards and set, it can produce a seamless finish between the boards, making the final wall or ceiling blemish-free before painting.

As the name suggests, joining compound is primarily used to smooth the joints and edges between separate plasterboards. That’s what the following instructions will focus on, although the compound can also be used to fill any other holes in the plasterboard.

What You’ll Need to Apply Joint Compound to Plasterboard

Applying joining compound to plasterboard is straightforward so long as you have everything you need. For a simple job – such as applying joining compound to the joint between two plasterboard sheets, you’ll need the following:

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Before we begin, it’s important to note that different joining compounds will have different instructions in areas such as drying time or when to apply the second coat. This guide provides some general expectations but we recommend always following the manufacturer’s instructions over the times provided here.

1. Prepare the Joint

First of all, make sure the joints are ready for the joining compound.

It’s recommended to dust the joints down first to ensure nothing inhibits the compound from sticking. Also, check that there is no more than a 1mm gap in the joint. If the gap is larger, you may need to fill it with some mortar first.

2. Cut Jointing Tape

Jointing tape – also known as scrim tape and usually made of paper – provides a crucial additional layer of strength between the separate boards. Measure and cut the jointing tape for the length of the joint you’re working on before applying joint compound.

3. Apply the First Layer of Joint Compound

Using your jointing knife or a scraper, apply a healthy amount of join compound to fill the joint. Work from top to bottom until the join is adequately filled and a layer of jointing compound covers the surface.

4. Apply the Jointing Tape

Apply the jointing tape on top of the wet joint compound, pressing it into place with your scraper or knife to remove any air bubbles from the joint.

Many jointing tapes will indicate which side should face into the joint compound – it’s usually printed-side down. Folding the tape in half can help to guide it down straight across the joint. Apply in the top corner first before smoothing down.

5. Apply a Second Layer of Joint Compound

Add a second layer of joint compound over the joining tape, this time covering a larger area to ensure the edges of the compound are as smooth as possible against the plasterboard walls.

When to start this step can vary significantly depending on your joint compound manufacturer. Some products will ask you to let the first layer dry before applying a second – this can take any amount of time from a few hours to a few days. Alternatively, some products will suggest applying the second layer before the first one dries.

6. Apply a Third Layer

After the second layer has dried, apply a third. As with the second layer, the third one needs to go wider than before and be smoothed out to create as seamless a finish between the wall and the compound as possible (it won’t be perfect until the next step).

7. Sand Down to Joint

Once the joining compound has finally completely dried, sand it down to create a smooth, seamless finish.

The joining compound only needs to be sanded lightly. Once everything’s smooth, dust it down. The surface should now be seamless and ready for painting or priming.

Hopefully, this guide will help you take on your next job with confidence. Need any more supplied for your project? See our plasterboard fixings at great trade prices.