Before you start: The important thing for a high quality polymer/acrylic render job over your Blueboard or Harditex base sheet is to get your joints flat to provide a nice smooth surface for your finished coats! The crucial aspect is side lighting from your exterior walls. You don't want to see any joints once the job is finished.
Clean surface down with a soft haired broom to remove all dust and dirt.
Note: Make sure Blueboard sheeting is not damp; if so allow to dry before continuing. Also, it is not advisable to apply the coatings if rain is imminent.
Cut a length of Naturatape, the length of the join that you are about to cover with Naturapatch Coarse patching compound, put it aside.
Apply a tight coat of Naturapatch Coarse with a broad knife, spatula or 300mm trowel to one joint on the Blueboard at a time. Now push the Naturatape into the join that you have just applied Naturapatch Coarse.
Now you can move onto your next joint, you are best to do the first coat on all the joints before starting on your second coat.
Once first coat is dry, scrape any trowel marks or dags off the first coat in preparation for the second coat.
Apply a second coat of Naturapatch Coarse with your broad knife, spatula or 300mm trowel.
Finally, run your trowel or broad knife back over the join to check that you are not leaving any high spots or lumps. It is better to be a bit shallow rather than proud when filling in your joins, you can always put more on but it is very hard to take it off once it is dry.
Now you can move onto your next joint.
Patch any dents, low spots or Blueboard recessed edges that are not getting taped, e.g.; expansion joints if you are not planning to use a plastic expansion joint, they are to be set only. Use your judgment as to whether it requires one or two coats after you have applied the first coat and allowed it to dry.
NOTE: Be careful not to tape any expansion joins. As they are to be cut at the end of the job. We would recommend using the plastic expansion joints as you get a much better finish, once you have filled them with the suitable sealer, see below under the headings Corner Beads and Expansion Joints and Final Work.
Allow to dry.
TIP: If you have a bucket of water and a scourer with you to dip your broad knife, spatula or trowel into and wipe with the scourer, keeping your tools clean and wet you will find that your trowel or broad knife will leave the Naturapatch Coarse a lot smoother and will not drag the joins out, as it can do when some of the Naturapatch dries on your broad knife, spatula or trowel. Also it will help keep your tools clean and save you time at the end of the day.
Corner Beads and Expansion Joints
Apply Naturatrim (2.5 or 3.5mm plastic external corner beads) to all corners and plastic expansion joints to expansion joints using a suitable construction adhesive, Liquid Nails or similar. Note: You can fit the corner beads and plastic expansion joint either prior to setting the joints or after you have set the joints, it is not critical what order, however before is probably just a bit easier. Just remember if fitting the corner beads and plastic expansion joints after setting the joints, make sure the Naturapatch Coarse on the joints are dry before fitting them.
Now you need to apply 2 coats of Naturapatch Coarse to each side of the beads to feather them out to nothing, so once the finish coats are applied you cannot notice where they have been fitted.
You do this by applying a first coat of Naturapatch Coarse feathering it from the bead to the Blueboard. You only want the first coat to be a maximum of 200mm wide, if you use a broad knife or spatula this will make it easier.
The second coat of Naturapatch Coarse goes over the first coat, however you want to make this coat wider so you achieve a more gradual feathering from the Blueboard to the bead. You will get the best finish if you use a broad knife or trowel that is about 300mm long, so that you can reach the whole width with one pass.
You will need a hawk and 250mm or 300mm trowel to apply the next two coats and a hard plastic float to finish them. You are best to have two people doing each coat, one putting on and the other following close behind, floating.
Apply one thin coat of coloured Alfresco Medium Sand polymer/acrylic render to the entire surface by working from left to right on the wall, if you are right handed and if you are left handed you will be best to work from the right to left. Using a steel trowel at approximately 45 degrees to the wall apply the Alfresco Medium Sand polymer/acrylic render, this enables you to be able to put it on quite tight, making sure you don’t put too much on. Put a hawk full of Alfresco Medium Sand polymer/acrylic render on the wall and then go back over it with an empty trowel, scraping off any access. Now you can move onto your next hawk full, while the second person starts to float what you have just put on.
Note: So if the acrylic render is dragging when you are floating, then you are going over it maybe a minute or two too soon or you are putting it on too thick. Your motto should be ‘the less the better!’ when applying acrylic render. You can easily put too much on this will result in the acrylic render dragging when you go over it with the hard plastic float.
This is exactly the same process as the first coat, however you can choose what finish you want either coloured Alfresco Fine Sand which is a very fine finish, with this finish you will need a 3rd coat to get a high quality finish. Or there is Alfresco Medium Sand which is a reasonably fine finish normally not requiring a 3rd coat, however if Blueboard is uneven this may also need a 3rd coat. The other option is Alfresco Graincoat polymer/acrylic render which is a bit more course finish also a bit more forgiving if your Blueboard is uneven as the larger grain fills a little, however if your Blueboard is very uneven this may also need a 3rd coat.
Third Coat if Required
This is exactly the same process as the first two coats. You use the same product that you used for the second coat, so if you used the Alfresco Fine Sand then you use the Alfresco Fine Sand for the third coat also. If you used the Alfresco Medium Sand then you use the Alfresco Medium Sand for the third coat also. Or if you used the Alfresco Graincoat then you use the Alfresco Graincoat for the third coat also.
If you haven’t fitted plastic expansion joints, then you need to cut the expansion joints where they are in the Blueboard, they will have been covered over with the Naturapatch Coarse and Alfresco polymer/acrylic render, but not the Naturatape. You do this by placing a long straight edge along the expansion joint and running a Stanley knife or half of a masonry blade along the straight edge, cutting right through the acrylic render. You ideally want about a 3 to 5mm gap.
Next, fill the plastic or the cut expansion joint with a suitable flexible sealer such as Shieldflex 1000 or Sikaflex. Paint flexible sealer to required colour.
TIP: If you have a bucket of water and a scourer with you to clean your trowel and hawk, keeping your trowel clean and wet you will find that your trowel will leave the Alfresco acrylic render a lot flatter and will not drag. Also it will help keep your tools clean and save you time at the end of the day!
A: It is not that easy to answer your question, for a number of reasons. One, we don't know what product was used to render your Blueboard.
Second, we don't know if the render was applied properly.
And thirdly, we don't know if the Blueboard was fitted properly, according to the manufacturer’s specifications.
To give you some direction though, if you read the ‘DIY Acrylic render on Blueboard’ instructions above, and can see that a step or two was left out by the applicator- eg, no tape was put on the joints before applying the finish coat, then you maybe able to determine the cause.
Another possible cause is that there is either no or not enough expansion joints put in the Blueboard. Each manufacturer has their own specifications, however normally they require expansion joints approximately every 5 meters vertically and about every 3 meters horizontally. If the manufacturer’s instructions are not followed then cracking will most likely occur.
A: Good question! The Shieldcoat Alfresco range has a binder ratio that is much higher than theirs (other brands). What does it mean that the binder ratio is much higher? It means that the Alfresco range holds its gloss and colour for a lot longer, even without the clear sealer over the top.
A: Yes you can apply acrylic render products straight over painted fibro or asbestos. You can follow the DIY Guide for Blueboard as detailed above.
Yes, you can apply acrylic render products straight over fibro. The manufacturer would recommend cleaning/removing as much of the previous flaking or loose paint because if it lifts it will take the render with it. A test patch is always recommended to test for adhesion.