Finishing off and skimming a plasterboard repair

[mikea]

I have repaired a hole in plasterboard with new board. the size of the repair is about 40 cm each way but not a regular square. I plan to skim over the repair to level it off and then replace the tiles. The result must be watertight for since it is in a shower area.

Does the skim plaster go straight on to dry board?

Should I fill the joint between the old board and the new before skimming? If so what with? Is a building adhesive suitable? Scrim tape does not seem to be the answer because of the existing plaster up to the edge of the repair. The gap beween the edges of the board is from 0 to 3 mm. Part of the repair edge lies very close to the joints between the tiles so I am concerned that any flexing between the old and new board could compromise the grouting.

[kbrownie]

Hi
The ideal way, would be to remove part of the plaster on the other side of the joint to allow a tape to be put on the joints then to skim over using two application of finshing. If you run a stanley knife about 30-40mm around the joint and use a scraper to remove plaster,there should be little disturbance to tiles and that will allow a tape to be applied. The tiles and grouting are the main factor as far as waterproofing the wall, if you also get a waterproof advhesive for fixing tiles that should do.
Good Luck KB

[mikea]

Thanks for that. However it leaves me with a bit of problem. I can remove about 25-40 mm of plaster around about 70% of the join. For the remainder, the edge of the old plasterboard is only about 5mm from the existing tiles. I am reluctant to attempt removing further tiles because of:
1) risk of breakage - no spare tiles and pattern long since discontinued
and
2) existing grout is very hard - original tiler made a mess with non-waterproof grout so had to replace it. He then went overboard with what I think is an epoxy based grout normally used for sterile surfaces like kitchen worktops and in hospitals. The only way I have been able to remove this is by grinding it out with a grout removing tool but because of the narrowness of the tile spacing (~2mm) and the hardness of the grout this inevitably also grinds away the edges of the tiles.

The back of the new board is very well supported with pretty solid battens or wall stud with no more than 100mm spacing between. Most of the problem area is actually directly supported be a wide batten glued and screwed behind. Unfortunately the problematic part of plasterboard joint will almost directly underlie the tile joint which worries me a bit because it is in a shower area. The question then becomes how essential is the scrim? Can I get away without it for at least part of the repair wihout turning it into a total bodge?

Thanks
Mike

[kbrownie]

Hi,
you can get away without it as the scrim tape is to prevent the area that is to be plaster or jointed from cracking as you area fixing tiles on to it, it is not really nessecary. I always do! but that's the way I am. If you still want to apply tapes 5mm on joint not ideal but should still be okay and prevent cracking on joint.
KB