Hardwall in one pass and very dry blocks

[akist]

I have had a few rooms and areas hardwalled and then plastered. All the different plasterers (about 4) applied the hardwall in one pass and then kept going over it to smoothen it out, but 99% of the hardwall material was applied in the first pass, thickness depending on the blockwork which is very badly done, usually anything from a few mm to 15+ mm.

The issue of the very, very dry blocks was approached in a variety and combination of contradicting ways, including hosing down the blocks with the garden hose, applying some or a lot of PVA, and using render mesh straight on the fresh hardwall. The last group of plasterers used render mesh on every wall, it comes in a roll I think 1m wide. Additional techniques were to apply PVA many times, apply PVA over the hardwall, and hardwall immediately while PVA still wet (so it bonds together), or wait half/one day before hardwall to allow the PVA to dry (so as to seal the blocks better).

But NONE of the plasterers tried to apply the hardwall in thinner coats, they all put as much as was needed on the first pass. Subsequent passes were to smooth out rather than add material. When I questioned why not go over it a couple of times with thinner coats they replied it was too much work and no one ever does that.

I believe (but could be wrong) that had the blocks not been so dry (thermalites) this would not be an issue.

As I still have a significant amount of walls to render/plaster could you please advise me how it should be done?

 

[Swwils]

There are a few key issues at play:

1. Highly absorbent substrate: The thermalite blocks are highly absorbent and can suck the moisture out of the hardwall too quickly. This can weaken the bond and create potential failure points.
2. Uneven blockwork: Poorly finished blockwork that's uneven can be difficult to cover with a single coat without exceeding the maximum depth specified by the manufacturer.

Here's a suggestion:

Preparation

1. Control suction: Use a product such as Febond Blue Grit from Everbuild. It is designed specifically to provide a high grip surface for rendering and to also control suction on difficult substrates. Apply it generously on the dry blocks and allow it to dry fully before applying any plaster.
2. Correct major imperfections: If your blockwork is very uneven, you might want to use a sand and cement render to level it out first, this will provide a flatter, more stable base for your hardwall. You could also consider the use of a bonding agent, like Febond PVA, which can be applied on the rough and uneven blockwork, left to dry, then the sand and cement applied.

Plastering

1. Apply hardwall in two passes: Contrary to what your plasterers have been telling you, it is entirely possible and often beneficial to apply the hardwall in two passes rather than one. The first pass is used to fill out most of the depth and provide a base. The second, thinner pass can then be used to provide a final, smooth surface. This approach can give the plaster more time to bond with the substrate and can reduce the chances of delamination and cracking.
2. Use mesh: If you're still experiencing issues with cracking or delamination, you can use a render mesh embedded in the first pass of hardwall to provide additional reinforcement.
3. Allow sufficient drying time: Finally, ensure each coat has adequate time to dry before the next one is applied. Rushing this process can lead to weaknesses and failures in the plaster.

Remember, each situation is unique and might require a slightly different approach. It is also crucial to follow the manufacturer's guidelines for product application, as failing to do so can result in failure.