Hello, Briefly: is there a way to obtain a small amount of plasterboard tiling primer without paying for a full pack? In more detail: I noticed that a tile on our shower wall was no longer flush with the rest of the tiles. I prized it up, it came off easily. Things don't look too bad: the paper was rotten but the plasterboard seems fairly intact. I've gathered I need to dry it thoroughly, prime it, dry it, then reattach the tile. I've got some adhesive and grout from a friend, but he didn't have any primer. I know I can't use PVA in the vicinity of the shower, and correct me if I'm wrong but I believe any water based primers are also inappropriate and it needs to be something like aquaseal - but that costs £10 per bottle and I don't fancy spending that to reattach one tile. Any suggestions where to get a small amount? Do any builders merchants sell it by weight where you can come with your own container? Do any manufacturers have anything like sample sachets?
Forget the primer. Buy a tube of some really sticky goo ('No More Nails' type - there's lots to choose from on Screwfix) and fix it back on with plenty of that, then grout it. The grout keeps the water out, not the adhesive. I take it you've got powdered grout, not that ready-mixed rubbish?
Thanks Joe, responses are not exactly flooding in so I appreciate yours. I did consider just skipping the primer so that's probably what's happening, unless somebody comes up with something better before Saturday. I'm a bit surprised you recommend sticky goo. Do you think that will actually stick to the plasterboard better than tile adhesive? And yes, I've got https://www.screwfix.com/p/buildfix-flexible-wall-and-floor-grout-2-5kg-white/70239
The grout (that's the right stuff you've got), as long as you get plenty of it in there and the other tiles are solidly fixed, will hold the tile in fine. The goo is just to hold it in place until the grout has set. It's certainly not a good idea to use it for lots of tiles, but for a one off, I'm sure it will be fine. Mind you, some types of goo cost more than a tenner!
https://www.screwfix.com/p/geocel-the-works-sealant-adhesive-white-290ml/72264 there are others. However, I'll make a bet with you. If one tile has come loose and the paper surface of the PB behind is wet and rotten, so is every tile around it. Plan to re-tile in the not too distant.