Internal Plastering

Brave Harvey

Just wondering if any one can tell me what the process involves in carrying out an internal plaster job. Is the wet plaster a 3 sand/ 2 cement / 1 lime mix. This goes on at a thickness of 15mm. Does this require a skim coat of 3mm on this then or is the skim coat only for plasterboard slabs? thanks for any help.

jack of all

I'm sure someone more expert will arrive shortly with a better answer, but I'll get the ball rolling for you! Internal plastering on bare blockwork background is a 3 coat job- scud and scratch coat in sand/ cement followed by gypsum skim coat. Scud and scratch coat will come out at about 10mm thick overall, before scratching and skimming will bring the final thickness up to approx. 12mm. Scud coat will be the strongest coat in any plastering job with subsequent coats getting progressively weaker (in terms of cement content) to avoid problems with cracking etc.

Brave Harvey

Thanks Jack thats sounds like your on the ball there. I was told that for a good airtight finish you would need a "Two Coat Wet Plaster Internal Finish". So from what you are saying, that would mean an initial scud, then a scratch coat of maybe 7 or 8mm, then another scratch coat of 7 or 8mm and finally a skim coat of 3mm. That maybe would end up with an overall thickness of 20mm. That sound right to you?

jack of all

The traditional 3 coat work I've suggested should be more than adequate (scud, scratch and skim coat), once painted,for normal concrete blockwork backgrounds, once care is taken with detailing at frames, floors, penetrations etc and in these instances you will need to use proprietary sealing/ air leakage tapes. It might be tempting to double up with an additional scratch coat but this will mean considerable extra work (and cost!) and increases in thickness, which may not seem important but will mean you may have to restort to off-standard door frames and linings etc. For example a standard internal door frame for 100mm (4") blockwork is typically 125mm x 44mm (finished size)- this equates to 100mm for blockwork plus 12.5mm for plaster finish each side. Increasing the plaster thickness, even by 8-10mm pushes the door frame size up and you may have great difficulty sourcing a 132mm or 135mm frame to suit.

Brave Harvey

Yeah good man your right there, the extra cost would be the issue along with the off standard door frames. Thanks for that. I haven't seen proprietary sealing/air leakage tapes for penetrations through a blockwork wall. Would you use an ordinary sealing tape? Tape to the blockwork and then plaster around the penetration, maybe nail a expanded mesh over the tape and plaster over?

I think if you are using an ordinary tape on a blockwork wall you will need a primer applied to the wall first to smooth out the surface?

jack of all

I'll be honest with you in that I don't have enough experience with those tapes to give advice, all I can suggest is you check out the manufacturer's web sites and look at the technical info, Pro clima being one I know of. Have you checked out the Construction & Planning forum here also? Some of the guys there will definitely be more up to speed on this.