I saw the tip from Rob Tester about mixing a little sand with PVA and painting on to create a coarse surface which any tile adhesive will have no problems sticking to? Can anybody supply for info:
a) Does this really work?
b) What would be the mixture ratio?
c) If I decided to paint this over my existing tiles do I still have to clean them with sugar soap first.
To give more information to rosebery's answer, you need to consider 3 things; first, are the existing tiles soundly fixed to the wall? If not, you are wasting your time by tiling over them. Second, what is the wall constructed of that is behind your original layer of tiles? All materials have a finite load strength which means that if you put too much weight on it, it is likely to fail. Consider the existing tiles and how heavy they are with the adhesive used to fix them. Add to that the weight of the tiles you wish to fix and the adhesive you will use for that. The grout also has a weight although we don't often add that in because we round our figures up. We calculate those weights to the square meter and then look for the strength of the wall material per square meter and compare them. If your tiles will be too much, then you know the risk you will be taking. Third, as has already been said, the existing wall needs to be spotlessly clean. To give you some examples, we tiled a bathroom last year where the wall tiles weighed 30kg per square meter plus the adhesive and grout. We had to completely remove the plasterboard and fix stronger material to the studwork. Another example, skimmed plaster (multifinish) is not as strong as bonding so, if you are building a wall that will tiled you might stop plastering at the bonding stage and tile on that if your tiles are heavy.
Grumpy Old Git
If "seem to be" is the operative given the option I'd still have 'em off and you'll not get me to say its OK I'm afraid if they only "seem to be". Actually you won't get me to say its OK either if you say they most certainly are LoL.
Anyway skimmed plaster can support 20 kg per sq m. If your old tiles plus addy and grout come to 10 kg per sq m and the new ones are the same you'll be coming perilously close to the failure point.