Hello I have built a retaining wall/raised garden bed with a bench seat using blue board sheets and we sealed the inside using the Sika Tite BE (water based). On the joins for the blue board we used the builders bog and the went over it with the Sila Tite BE. Now that we are going to paint the render on using the Dulux texture medium cover we ran into a problem as when I went to purchase the paint the lady at the paint shop said that nothing will stick to the bitumen paint. We then placed 2 coats of the Zinsser cover stain (oil based) on top. It said touch dry in 30min and reapply in 1hr, as I was applying the second coat I had leaned on a small part of the previously painted surface and it had come off down to the bitumen paint. Does this mean it hasn’t taken and won’t stick and seal? Or do I just need to wait longer? Also will this product seal and allow me to paint the render on top? If not what should I do now to be able to paint the render??
Welcome to the Bunnings Workshop community @Laura_18 . It's fantastic to have you join us and many thanks for your question on applying textured coatings.
There are a couple of issues here, the first being, as you've now discovered, that very few coatings adhere well to bitumen. It's an extremely difficult substrate for paints and primers to bond to, and bitumen is generally not listed as a recommended surface for most primer systems.
There is a lot of conflicting information available regarding whether water-based, oil-based or shellac-based primers perform best over bitumen. However, most manufacturers either don't recommend application over bitumen at all or advise conducting a compatibility test first. Unfortunately, it sounds like your test may not have been successful, as the Zinsser Cover Stain appears to have lifted back to the bitumen coating.
Before writing it off completely, I'd allow the primer additional curing time. While it may be touch-dry within 30 minutes and recoatable within an hour, full cure and adhesion development can take considerably longer. If the coating is still soft or can be easily removed after 24–48 hours, that's a strong indication that it hasn't bonded properly to the substrate.
Could you tell us a little more about where the Sika Tite BE has been applied and share a few photos of the project? That will help us determine the best path forward.
If the primer ultimately proves incompatible with the bitumen coating, one option may be to fully waterproof the existing sheeting across its face and then mechanically fix a new layer of suitable sheet material over the top. You could then apply the Dulux Texture Medium Cover and finishing system directly to that new surface, avoiding any adhesion concerns associated with coating over bitumen.
With a few photos and some additional details, we'll be able to provide more specific advice on the best way to proceed.
Please let me know if you have any questions.
Mitchell
Hi Mitchell, I know now but the paint specialist I spoke to at Bunnings said to use the Zinesser one which I’m now thinking I shouldn’t have as it was oil based on top of water based?
This is the project and had just sealed the joins, I don’t have an updated photo with the primer but it’s the exact same just with the primer directly on top of the bitumen paint
Hi again @Laura_18,
I've got in contact with the Sika technical team. Their advice is to use a water-based primer, as they believe the solvent in the bitumen coating could be interfering with the oil-based primer curing. As you've mentioned, it would be best to use a water-based primer over the water-based bitumen paint. At this stage, since you've coated everything, it might be worth waiting 48hours just to see whether the primer cures and adheres correctly. After that time, apply some sticky tape across the primer and push it down. Rip it back off. If it pulls off the primer, it indicates that it has not bound correctly. Mitchell
thank you so much Mitchell for checking with them! I will do that with the sticky tape in a few areas to be sure! If it comes off with the tape even slightly would it best best for me to scrape it all off using a paint scraper or similar right down to the bitumen paint, wipe it down with mineral turps and leave dry for a day, apply the water based primer and then put on the render
And if it’s dry will I apply the water based primer on top of the oil based one and then just put the render on top? Which water based primer do you recommend? I’d prefer to have a stain blocking in it to hopefully prevent the colour of the bitumen paint coming back through. Would the Zinsser B-I-N be ok as some people have said this is what I should’ve used in the first place? But that is shellac based I believe not water based?? If not which water based do you recommend
It's a bit difficult for me to give comprehensive advice here @Laura_18 because none of the primers we're discussing are specifically recommended for application over bitumen, nor are they really compatible with each other. That's usually because either they haven't been tested in that situation, or the manufacturer has determined they're not compatible.
The only guidance I can really go on is the advice from Sika, who recommend using a water-based primer. Unfortunately, most stain-blocking primers are either oil-based or shellac-based rather than water-based. Anecdotally, I have heard of shellac-based primers being used successfully over bitumen, but it certainly doesn't confirm compatibility with water-based bitumen variants.
We also need to consider the topcoat. The Dulux texture/render products are designed to be applied over porous cementitious or masonry substrates. They aren't designed to be applied over primers, particularly in situations like this where there are multiple coating layers involved.
As for primer recommendations, I can't confidently recommend a specific stain-blocking product over bitumen because there simply isn't any tested guidance that I'm aware of. While Zinsser B-I-N is a shellac-based primer and is an excellent stain blocker, I can't say with confidence that it's suitable over bitumen or that it would provide a compatible base for the texture coating. Dulux certainly doesn't recommend their texture coating over a shellac-based primer. In saying that, they also don't recommend their product over the current oil-based primer.
Regarding applying a water-based primer over the existing oil-based primer and then applying the render, this is where we start getting into a system that contains multiple products that may not have been tested together. We would potentially have the original bitumen coating, an oil-based primer, a water-based primer, and then the texture/render coating on top. Any one of those layers could have compatibility, adhesion, or flexibility issues with the layer above or below it. Even if each individual product performs well on its own, that doesn't necessarily mean the complete system will perform reliably long-term. That's why it's difficult for me to confidently recommend building up additional layers over the existing coatings, particularly when the manufacturers haven't tested or approved those combinations. The more products we introduce, the greater the risk that one of the interfaces between coats becomes the point of failure.
The ultimate answer is that the bitumen coating really needs to be removed and the texture/render applied directly to the cement sheeting as it was originally designed to be. That is the only approach where the system is being used in accordance with the manufacturer's intended application. Mitchell
