'How to' Guide: Spray Finish Acrylic Render on Blueboard with Hopper Gun
Before you start: The important thing for a high quality polymer/acrylic render job over your Blueboard or Harditex base sheet is to fit the board according to manufacturers specifications and to get your joints flat to provide a nice smooth surface for your finished coats! The crucial aspect is side lighting on your exterior walls. This is when sunlight shines down the wall from a very slight angle, which accentuates any slight hump or hollow by casting a shadow. You don't want to see any joints once the job is finished. So make sure your frame is flat before you fit the blueboard!
The important thing for a high quality polymer/acrylic render job over your Blueboard or Harditex base sheet is to fit the board according to manufacturers specifications and to get your joints flat to provide a nice smooth surface for your finished coats!
Preparation work
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.
Taping & Base Coating Joints
Cut a length of Naturatape, the length of the joint 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 trowel (that is just wider than the recessed part of the joint, this will help you not to put too much on causing wastage and more work for you, having to clean off the unwanted Naturapatch Coarse on the blueboard) to one joint on the Blueboard at a time. Now push the Naturatape into the joint that you have just applied Naturapatch Coarse too. Clean off any dags or excess Naturapatch Coarse that may be either proud or you have smeared beyond the recessed area of the joint. The Naturapatch Coarse CANT BE LEFT PROUD OF THE BOARD as this will cause you a lot of problems later.
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. Hopefully you have not left too many!
If the Naturapatch Coarse has dried a bit shallow you can apply a very thin second coat of Naturapatch Coarse with your broad knife, spatula or trowel. Before the Naturapatch Coarse dries, run your trowel or broad knife back over the joint 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.
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, no jointing tape! Use your judgment as to whether it requires one or two coats after you have applied the first coat and allowed it to dry. Remember, it is better to be a bit shallow rather than proud when filling in your joints, you can always put more on but it is very hard to take it off once it is dry.
NOTE: Be careful not to tape any expansion joints. As they are to be cut at the end of the job. However, 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.
Base Coating 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 of this width this will make it easier. Scrap off any excess or dags from the edge of the corner bead and also from the blueboard where you have feathered out too, as you go.
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 bead to the Blueboard. 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. Scrap off any excess or dags from the edge of the corner bead and also from the blueboard where you have feathered out too, as you go.
Base Coating Nails
Apply a tight coat of Naturapatch Coarse with a broad knife or spatula that is about 100mm wide. Feather the Naturapatch Coarse from the nail out to nothing. You can achieve this by pivoting your broad knife or spatula on the nail and press firmly on the blueboard with the outer edge of your broad knife or spatula and running around the nail in a circular motion.
Apply a second coat using the same method as the first coat. The only difference is use a wider broad knife or spatula to feather the patch out further.
Smooth Coating Joints, Corner Beads, Expansion Joints and Nail Patches
Note: 'When applying your finish coats with a hopper gun it is very important to finish the joints, corner beads, expansion joints and nail patches with at least one maybe two coats of Naturapatch Smooth to get as smooth as possible finish before applying you finish coats, as the finish coats do not hide any imperfections that are in the joint coats. Remember, the better the base for your finish coat the better the finish product!'
Apply a thin coat of Naturapatch Smooth with your broad knife, spatula or trowel approximately 25 to 50mm wider than the existing base coat of the joint, corner bead, expansion joint or nail patches. Try to feather the edges of the Naturapatch Smooth as much as possible. Even though Naturapatch Smooth is sandable, the less you have to sand the better, as it does dry pretty hard.
Once first coat of Naturapatch Smooth is dry then run your trowel or broad knife over the joins to scrap off any high spots or lumps, hopefully there isn't any!
If it looks like the first coat of Naturapatch Smooth has shrunk back a bit or is a little shallow then you may need to apply a second coat of Naturapatch Smooth. If you are having trouble feathering the edges because the Naturapatch Smooth is a bit thick then you can add up to .5% water to allow you to put it on a bit thinner.
After Naturapatch Smooth is dry then you can sand it back using a hand sander with 150 Grit sandpaper.
TIP: Do not leave Naturapatch Smooth to long, eg days, before sanding as Naturapatch Smooth gets harder the longer you leave it.
When you have finished sanding the Naturapatch Smooth, broom it down with a soft broom. Then check that the joints are flat, smooth and without any holes or dips in the surface. If it all looks good, you are now ready to get your hopper gun out and start you finish on!
First Coat
Firstly, you will need a hopper gun and an air compressor that can pump out at least 10 cfm (cubic feet a minute) with a regulator. The pressure you need to set you regulator will vary on your compressor, hopper gun and the product you are applying, so start with a very low pressure of about 25 psi and then gradually turn it up until you start getting an even flow coming out of the hopper gun spray tip.
Note: If it is your first time using a hopper gun, then we would suggest that you practice on some spare pieces of blueboard until you have the confidence and a good feel for applying it with the hopper gun. This can be easily carried out by one person, but if you have another person helping just load your hopper it can be a help.
Apply one thin even coat of coloured Alfresco Fine Sand or Alfresco Medium Sand polymer/acrylic render to the entire surface by working from left to right (or right to left which ever suits you most) on the wall in a systematic way so as not to miss any areas. It is best to keep a wet edge. For best results apply the finish coats either in the shade or avoid the heat of the day so that you do not get the working edge of the coat drying before you have finished spraying. If you do get a few areas where the working edge did dry before you had to spray up to it and there is a little bit of misting, don't worry the second coat will cover it. Just take note of it so that you don't have it happening in the second coating.
Second Coat
This is exactly the same process as the first coat. However, you need to put on this coat a little thicker just to give the even coverage of the grain in the coating.
Third Coat if Required
This is purely dependant on personal skill and personal preference, as to how even a coverage you have been able to get on the first two coats and also what type of finish you are after.
The third coat is exactly the same process as the first two coats.
Final Work
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, Naturapatch Smooth 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.
For more job specific guides visit our pages on roof paint, concrete paint and heat reflective paint.
Spray Finish Acrylic Render on Blueboard with Hopper Gun F.A.Q.
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.
Q: Can you please tell me why the render on our Blueboard facade has cracks where each join is?
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: 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.
