What to putty on aerated concrete/old fix before tiling on top?

[Andreas Lundgren]

Hello!

I have removed tiles in an old bathroom. The tiles were fixed with adhesive on lightweight concrete (without waterproofing), so now there is a mixture of lightweight concrete and old adhesive left. The remaining adhesive cannot be removed.

Now I need to plaster over it so that it can be waterproofed and then retiled. It needs to be something that

1. Can support tiles
2. can be made smooth by a layperson.

At Beijers, I initially got Weber's repair mortar and a primer called "Floor Primer." When I called to double-check if I really needed the floor primer, the person who answered said I could instead just dampen the wall and then use Gypsum Naturgips (Weber's gypsum product). It was said to be easier to work with and just as durable.

Since I have now received different advice, I just wanted to double-check here,

• Can Gypsum support the weight of tiles?
• Do I really not need to prime?
• Is it okay to apply waterproofing (I plan to use LIP's cement-based 2-comp with sheets) on a gypsum-based surface?

 

[anders07]

I had a similar problem... I first tested with gypsum plaster (Gypsum) to make it smooth, and it went fairly well, but it took a very long time. Then I noticed the wall became wavy, and I had to start fixing the waves. I fixed the remaining three walls by "gluing" Jackon boards with gypsum plaster and adding 8-10 nail plugs with a large washer in each.

Use a plant spray can and water quite thoroughly before applying the gypsum plaster. No primer!

 

[anders07]

And I can add that the first wall took about twice as long as the other three combined...

 

[Andreas Lundgren]

So it's not as simple as applying putty once horizontally first, then once vertically, and then it's good? Lucky to be naive; otherwise, one would never start these home projects.

Thanks for the tip, I'll have to learn from my own mistakes, but maybe I shouldn't buy more than one bag of plaster to start with. ;-)

 

[Steinmeister]

I had leveled everything with tile adhesive, it sits well and is easy to work with.

 

[anders07]

I would level everything with tile adhesive, it sticks well and is easy to work with.

A bit tricky with aerated concrete and adhesive. Some "burn" the aerated concrete from what I've heard and it can lead to it starting to come loose. Gypsum plaster is safer in that aspect as long as you water it sufficiently first.

 

[Steinmeister]

http://www.norgips.se/montage/vaggar/renovering/index1,206.htm

 

[anders07]

So, it's not as simple as putting filler once across first and then once vertically, and then it's all good? Lucky to be naive, otherwise, I would never have started such home projects.

Thanks for the tip, I have to learn from my own mistakes, but maybe I shouldn't buy more than one bag of plaster to begin with. ;-)

Yes, you have to be naive as hell... otherwise, I would have taken all the tools to the recycling center a long time ago!

If you want it flat enough to lay tiles on later, you have to use render guides. It's a bit tricky with waterproofing (with foil) on plaster because there are quite a lot of air bubbles, I noticed... I used Kiilto's system. I regret not putting boards on the fourth wall as well since the waterproofing there didn't "take" as well as on the boards, but that wall consists of about 70% of a large mirror, so I don't have to worry about tiles falling due to any air bubbles...

If I may make a recommendation, put up boards, you will save a lot of time and especially lots of swearing...

 

[anders07]

[link]

There you go, contrary to what I've learned (and read)... I've "glued" lots of plasterboards (and the Jackon boards) with gypsum plaster to my lightweight concrete walls (both Ytong and blue concrete) and they've stuck like a rock and have been doing so for almost 3 years now.

I guess the difference might be that you may not need to water if you use adhesive, but both seem to stick well enough...

 

[Andreas Lundgren]

The old tiles were fixed with adhesive, and it holds harder than the aerated concrete. If I try to remove it, the concrete breaks before the adhesive comes off. However, I don't know if there was primer in between or not.

Thanks for all the tips, it bothers me that I'm going to lose a whole cm of floor space, but I guess it's mostly psychological. I should probably just go with the boards directly without fussing!