How to sand fibre cement joint compound?

[swalker14]

Hi,

I’m renovating a bedroom and have used 4.5mm fibre cement sheeting to replace a damaged wall and match in with the existing sheeting width. I’ve used Dunlop Fibre Cement Sheet Joint Compound and Builders Edge Joint Tape to set the joins. However, when trying to sand back the joint compound to a smooth finish, the 150-180 grit sandpaper is getting torn up. I’ve also tried 150 grit slotted screen mesh. It happens whether I’m hand sanding or using an orbital sander. Is there specific sandpaper or sanding discs that I should be using for this product? 

Appreciate any advice or insight! 

Steph

[JacobZ]

Hi @swalker14,

Thank you for your question and welcome to the Bunnings Workshop community, it is fantastic to have you with us.

Generally speaking, Dunlop Fibre Cement Jointing Compound is not used as a topcoat, rather as a jointing compound intended to give the join between two boards as much strength as possible. This means it is designed with strength in mind rather than the ability to sand to a smooth surface. 

It is possible to sand it to a smooth finish, but you will likely destroy a lot of sandpaper in the process.

My suggestion is to get rid of any hard edges on the jointing compound and apply a coat of Less Mess Topping Compound over the top that is wider than the first coat of jointing compound.

Once this is applied, it will be far easier to sand to a perfect finish than the jointing compound.

I'd suggest having a look at How to plaster over new Gyprock plasterboard for some good advice that could assist you with this job.

Let me know what you think and if you have any further questions, please don't hesitate to ask.

Jacob

[swalker14]

Hi Jacob,

Thank you for your reply and advice. Any tips on how to get rid of the hard edges effectively or take off the product and use Gyprock base and top coat instead? 

Thanks,

Steph

[JacobZ]

Hi @swalker14,

When I scrape plaster compounds to remove hard edges, I use a combination of a high-quality metal joint knife such as this Hyde 4" Plasterboard Joint Knife run along the length of the joint to scrape the edges and then I feather them with a Coarse Sanding Sponge.
 

There's no real need to remove the jointing compound as this is its intended purpose. You don't need it to be perfect, but if there are obvious lines and humps in the jointing compound, your topcoat will need to be thicker and wider to feather the edges out and make it imperceptible, so any improvements you can make in the first coat will make your life easier down the line. 

The trick with jointing sheets is to have each coat wider and smoother than the last. My plaster joints are often 300-400mm wide by the final coat so don't try and go too narrow. Using a wider knife such as this Kango 200mm Taping Knife will help you achieve a fairly smooth topcoat that is wide enough to cover the jointing compound, the sanding is then the final step to make it perfect.

Stick with it and don't get disheartened, you're on the right track, there's just a few extra steps on top of what you've already done.

If you run into anything tricky in the process, please don't hesitate to reach out, I am more than happy to assist further if required.

Jacob

[jan7]

We have used cement fibre jointing compound to fill gaps on our ceiling. Hubby tried to sand with 60 grit sandpaper with a  full boar electric sander and it will not sand down! It is very rough, maybe he applied too much. Should you only use a little bit? How do we sand it down to apply primer and paint? 

[harman198]

Hi @JacobZ ,

We've put some 4.5mm fibre cement sheets up on the outside of the house to replace asbestos. Is the Less Mess Topping a compound suitable in this use case? If not, could you please recommend a solution to get a smooth finish on the fibre cement joins that will be suitable for external wall?

We also have made a mess of some joins with the Dunlop Fibre Cement sheet compound 🫤

Thank you

[JacobZ]

Hi @jan7,

Thank you for your question and welcome to the Bunnings Workshop community, it is fantastic to have you with us.

Don't be disheartened, there's not too much there and it actually looks like a decent first coat, there are just a couple more steps beyond this to get the best result possible.

As you'll see in the discussion above, fibre cement joint compounds such as this Dunlop Fibre Cement Jointing Compound are not intended to be used as topcoats and are designed for their strength, consequently this makes them extremely difficult to sand.

I'd suggest grabbing a high-quality steel joint knife such as this Hyde 4" Plasterboard Joint Knife and scraping the joint compound. You are aiming to remove any obvious raised sections and dull the transition from joint compound to sheet as these will show through the next coat.

If you only have a small area, you can use the Gyprock CSR 2.25kg Less Mess Topping Compound that I've mentioned above, however, it looks like you're going to need a bit more than this small bucket, so I'd suggest grabbing a bucket of Gyprock CSR 20kg Jointmaster Topping Compound instead.

Follow my advice above about applying and sanding the topping compound.

Let me know if you have any further questions.

Jacob

[JacobZ]

Hi @harman198,

A warm welcome to the Bunnings Workshop community and thank you for your question.

Unfortunately, the Less Mess Topping Compound is not suitable for external use.

In external applications, I am not aware of a plaster compound that can be used on walls. 

To get a smooth surface, you'll have to use a fine coat render such as this Dunlop Fine Coat Render, however, you will need to render the whole wall to get a uniform look.

Check out How To Render for some guidance on this.

Let me know if you have any further questions.

Jacob