Most commonly encountered backgrounds in re-skimming jobs will need no pre-treatment before using our ThistlePro UniFinish. However there are a few that will need some preparation first and a very small number where Thistle UniFinish is not suitable even with pre-treatment.
Backgrounds requiring no pre-treatment:
All backgrounds need checking to see that they are clean, sound and free from dust. After checking this, no pre-treatment is needed for:
- Most paints, including matt, silk, vinyl, eggshell emulsions, kitchen/bathroom paint
- Walls stripped of wallpaper (with minimal wallpaper adhesive remaining)
- Painted texture compounds
- Undercoat plaster, except if very high suction (see below)
- Skim finish plaster
- Tile adhesive
Some preparation or pre-treatment is needed for:
- Very dirty backgrounds
- Loose or friable surfaces – these need to be stripped back to a sound surface. Loose layers can be identified by a hollow sound when tapped, and friable surfaces are problematic if it is possible to keep rubbing dust off them by hand. After stripping them back, the sound surface can then be assessed – in many cases the required plaster thickness will necessitate dubbing out, patching or complete re-plastering with a new undercoat before refinishing.
- Extremely high suction backgrounds – e.g. some lime-based or sand/cement backgrounds – these need suction control, which can be done with one application of Thistle GypPrime (diluted as required – it is intended to soak in and block pores within the background, not to remain at the surface). PVA may also be suitable. If this suction control is not done and the first application of Thistle UniFinish crazes severely, the suction should be controlled before continuing. For tips on dealing with more mild suction however see below.
Unsuitable backgrounds include:
- Unpainted texture compounds – these can soften significantly when wetted, leading to adhesion failure within the compound or of the whole compound layer from the original substrate
- Self-cleaning or anti-graffiti paints – these are often designed to not fully harden, or to have a deliberately water-repellent or weak surface layer so that materials that stick to them are removed easily, so they are not suitable for plastering
- Surfaces where there is flaking or delamination between layers, and poorly bonded, friable or dusty surfaces – clearly there is a high risk of failure unless such materials are removed
- Unpainted Artex and other texture compounds
What about backgrounds not listed here?
Our recommendations and advice come from controlled testing and “real-world” trials across many sites and situations. In re-skimming work, other backgrounds will inevitably occur but we have not experienced them often enough to have sufficient data to support a clear recommendation. All we can suggest is that customers apply common sense and caution – e.g. it may be appropriate to test a small area in some cases, and to remember that it is not enough that Thistle UniFinish adheres to the surface itself – the whole system of previous layers needs to be stable and to hold together during and after refinishing. If there is doubt about whether it will, then removal or overboarding with a system like GypLyner UNIVERSAL should be considered.
What about painted Artex and other texture compounds?
If painted they are probably suitable, but note the following:
- They still need to be checked for cleanliness and soundness
- Deep texture patterns will be more difficult to cover than shallow patterns
- Scraping the peaks off a deep texture before plastering is not recommended for two reasons: 1) if the compound is old enough it may contain asbestos and 2) scraping removes all paint from small areas, exposing unpainted compound
- While Thistle UniFinish will adhere to the painted surface, no-one can guarantee that failure within or between earlier layers won’t occur, e.g. between texture compound and the original substrate, when more weight of plaster is applied.
Please refer to White Book Chapter 07 Section 02 Page 04 and Page 07 for further details.