Not quite Jay. Prime concrete only if it's "hungry". For example, pour a cup full of water on to the slab you are tiling. If there's still some water sitting on the concrete after a minute, you don't need to prime it, just wash the dust off. Can't think of another way to describe it.

yeah know what your saying Frank just getting a bit confusing so if the plaster board must be primed mainley due to dust ect then the concrete floor which is were most of the plaster dust lands doesnt just smear it around with water and alls good

nah think ill stick to priming all with an acrylic primer watered down i hear you but im old school and never had a prob to date (if it aint broke leave it be)

each to there own thats what we like about tiling different ways

Hi there , First time replying to post . Have been tiling for 36 years though. Yes I have to agree with Frank if the concrete is too hard then priming isn't a good idea , the modern glues can deal with it. As for FC and plasterboard I prime to make sure the dust is no longer a problem as well as giving more time for the glue to set properly.

Thanks

Originally Posted by FrankM

Interestingly, for an adhesive to comply with the new adhesive standard, they are all tested on unprimed surfaces. Although, concrete is one thing, if your going to use a cement based adhesive on FC wallboard or FC ceramic tile underlay, priming is a really good idea.

You need to remember that the adhesive test is done on a standard block of concrete with a known porosity and is essentially "perfect". It is imported from Germany and is the same accross the world to ensure you are only testing the adhesive.

You are better off priming every time. Tests are conclusive and you will get a better bond. The best primer will depend on the surface. Only use acrylic / penetrating primers on porous surfaces.

A super porous surface will benefit from a wet sponge before priming to reduce the primer "flash" drying.
Sure, if you prime a non-porous surface with a penetrating sealer you will reduce the bond. But it is not the right primer for the job.

If your surface is very porous and you don't prime the you risk drying out the adhesive before it hardens properly (crumbly adhesive).
If it is not porous (steel trowlled concrete) and you don't prime you will get a weak bond between the adhesive and the surface ("sticks to the tile but not to the floor").

Surface preparation is probably the most important step - if you don't prepare the surface then you are relying on the surface as installed (and can you be sure the concreter has not taken any shortcuts !!).

I look at it like laying the foundations of a house - if you take short-cuts and it doesn't work - it doesn't matter how well built the rest of the job is.

Remember that as soon as you tile onto the surface you accept that it is suitable for tiling and are liable for any issues. No matter how bad the previous tradie messed things up for you.

So even if you only get pinged for 1 job in 100 - how much will it cost you? Primer is not that dear per m2 - so think of it as cheap insurance.

Remember also that the primer will also seal in any minor dust you didn't get off during the clean-up step.

Always prepare the surface and always prime (just use the right primer). Not worth the risk not to.

Originally Posted by FrankM

Not quite Jay. Prime concrete only if it's "hungry". For example, pour a cup full of water on to the slab you are tiling. If there's still some water sitting on the concrete after a minute, you don't need to prime it, just wash the dust off. Can't think of another way to describe it.

I Disagree. What happens if there is a weak laitence layer just on the surface because the concreter added water during finishing. If that layer delaminates then your whole job is at risk.

You also need to make sure that your tile adhesive is suitable for non porous surfaces (don't choose the cheapies).

If your water "beads" on the surface then you may also have a curing compound or realease agent on teh surface. This will affect the bond. Woudl you be willing to guarantee that there is not?

One job that you have to go back to fix is worth quite a lot of your time and money. How long will you have to work to make that up on jobs where you are getting paid?

Originally Posted by jay

yeah know what your saying Frank just getting a bit confusing so if the plaster board must be primed mainley due to dust ect then the concrete floor which is were most of the plaster dust lands doesnt just smear it around with water and alls good

nah think ill stick to priming all with an acrylic primer watered down i hear you but im old school and never had a prob to date (if it aint broke leave it be)

each to there own thats what we like about tiling different ways

Priming board is not only about the dust. Priming with a suitable primer will strengthen the board. So if the job fails it will rip the board apart (rather than "pop" off the surface).

This is the case with any porous board (including fibre cement)

Hi Bosas,

I'll try and cover your posts in order from top to bottom without throwing in a heap of quotes.

Although some test slabs are in fact imported from Germany, there is no requirement to do so. The Australian (and international) Standards describe how they are to be constructed and the performance requirements these slabs have to meet. They do not represent the perfect concrete, as concrete formulations will vary from site to site depending on the engineering requirements for that job. The test slab construction is standard across the board, as is the test tile that is used, because if they weren't all testing would be not comparable and meaningless.

I am yet to view any internationally recognised testing which conclusively or otherwise shows that priming every substrate will increase the bond. However, I believe that this testing should be and needs to be done.

Steel troweled concrete can still be porous. If you are referring to concrete which has been troweled to a highly dense and almost polished finish, then it is not a substrate suitable for tiling and scabbling or grinding should be considered.

A laitance layer will not determine whether you should prime or not, as it needs to be removed in any case. The same goes for curing compounds, they need to be removed. So neither laitance nor curing compounds will have any effect on priming.

Most adhesives tested to As 4992 are likely to be suitable for surfaces of low porosity as the test tile has a water absorption of < .05%, but as I said, you should consider grinding or scabbling highly dense concrete. Even some of the "cheapies" may actually be better in some cases than an expensive adhesive, it depends on the job. Read the data sheets.

Priming is never about the dust, it is about water absorption. Dust will still accumulate on top of a primed area as you tile it and it needs to be constantly cleaned.

Be careful what you prime, and don't confuse a primer with a bonding agent, they do different jobs in different circumstances.

Be careful what you prime, and don't confuse a primer with a bonding agent, they do different jobs in different circumstances.

interesting quote Frank never realy thought about it that way

Possibly jumping in a little late here...............
I use the following principle, I prime if the substrate is suspect or can flake tear or separate. e.g. Powdery/flaking concrete, plasterboard & masonite or chipboard (vertical panels). Also if Im fixing heavy tiles to cement sheeting. Good concrete should never be sealed, infact i recommend etching to remove latence. Although not as common these days, a light acid wash on new concrete is one option if it is rinsed very well. I use a 3000psi pressure cleaner with turbo head for outdoor work.
The thing that annoys me, is tile shop sales people that usually have never laid a tile, are the ones supplying majority of the technical info!

Originally Posted by zimmy

Possibly jumping in a little late here...............
The thing that annoys me, is tile shop sales people that usually have never laid a tile, are the ones supplying majority of the technical info!

Couldn't agree more. One comment about your post 'though, masonite is not a suitable substrate for tiling, ever.