The short version: I need to (re?) seal a plywood base to re-install some tiles on a wetroom shower floor. What paint-on products would you recommend? Also, is there a sealant that can be painted over small (3") natural stone tiles to prevent water getting through/round them.

The long version (look away now if bodgery offends you): I recently bought a refurbished house the first-floor bathroom of which includes a wetroom style shower. The walls are large travertine tiles, the floor is 3" natural stone tiles on a mesh/mosaic backing. The grouting between the floor tiles is not great and has started to crack. The problem is worst around a few (only 8) of the tiles that were not seated properly and so rocked when you stood on them. The grout had completely disintegrated and there was very obviously water underneath these so I've removed them with a view to re-bedding them and regrouting. They appear to be laid direct onto plywood and I have no idea if this was sealed originally, there is a black substance on it but this may be discolouration due to water penetration (the ply appears sound and not soggy). I'd like to make sure it is sealed before I re-bed the tiles. What would you recommend? Please bear in mind that this is only a very small area (less than 1 sq ft) so a membrane is impractical, I need a paint-on solution. Having reseated these tiles and regrouted, is there anything that I can paint over them to help stop water working its way through the grout? The floor was painted with travertine and tumbled marble impregnator but I understand that this is to avoid water degrading the tiles themselves. I'm looking for something that will form a seal over the tiles and grout.

Disclaimer: The workmanship on the house is not top-rate so I fully anticipate having to redo the wetroom in a couple of years with a proper (Tuff2 or similar) tray and membrane-tanked walls. For now the floor seems pretty rigid, so I don't think the whole thing is flexing catastrophically, and I don't have the time or money to get this done.

Thanks in advance for your help

Welcome to the forum


you say that a membrane is not an option, is this because you dont want to lift the existing tiles? I think you will have to, applying sealer to just part of the substrate is not going to work, water will just find another route through to the ply. The ply itself may be the cause of the problem if its not absolutely rigid, any deflection at all and the tiles will pop off. The black mark is probably damp and if so this needs treating, it should be cleaned with bleach to kill off all spores and allowed to dry.
Remove the trav and do the whole area from scratch, incorporate a membrane that comes up the walls to a height of at least 1.6 mtrs, reinforce all corners.
Its not what you want to hear but a quick fix will just come back and bite you and make you put your hand in your pocket again.

Thanks diamondtiling, that was exactly the response that I was expecting. I completely agree that I will have to get this redone in the (fairly near) future. I hate doing half a job on this, and I hate even more that it was done poorly in the first place. However, I really do need to do as good a temporary fix on this as possible for a few months before sorting it properly.

To clarify, my reason for not using a membrane is purely because I don't want to lift all the tiles. Most of the tiles seem solid but water may be finding its way into the substrate sround them, albeit very slowly. I only removed the tiles that were moving so much that the grout around them had completely disintegrated. These tiles obviously had standing water around them (a bit like a loose paving stone that shoots water up your trouser leg after a rainstorm). If I can reseat and regrout them decently it will hopefully make the shower usable for a while before tanking and tiling it properly. Sealing the ply underneath is just to make this temporary fix as good as possible.

So I really want to know if there's any paint-on products that will seal the wood in the same fashion as a membrane.

You can buy liquid tanking kits.. this will seal the ply from water ingressing through the top but not anywhere else it is getting in..

I agree with Dom above.. not a perfect repair job.. it could fail in weeks..

For cement based grout to crack and fall out is either wrong grout or deflection in the substrate or even swelling of the ply.... water will not effect it to the state that it cracks etc..


The sooner you can get it done properly then the less damage you are doing to the floor substrate/joists and ceiling underneath.

Damp getting through will cause rot/mould etc and untold future problems.

Hi and welcome

Thanks for the welcome!

A quick bump for this thread. Also, looking here:

Tanking Shower

suggests that Ardex WPC is roughly what I want. Also apparently, Bal, PCI, Granfix and Weber do something similar. The PCI one is called lastogum, any ideas what the other ones are called?

For adhesive, obviously I need something flexible and waterproof (which is hellish difficult to find in your average DIY store). Would BAL Mosaicfix be ok?

These are products that are water based acrylic opaint type product which stop water penetrating through tiling but;
They do not stop water/moisture coming up from beneath and the substrate has to be sound and DRY before application.

hi as above

to the forum

you are wasting you time the job was never right in the first place and will not be right if you patch it .take it up and do it right !! will save a lot of time and money. the guys on here give good instruction on how to tank a wet room use it..