wall repairs - bonding and rendering

[DIYDouglas1]

I'm in the process of repairing a wall which requires a combination of large areas needing rendering and smaller ones that require patching-its a bit of a mess to be honest.

One area requires rendering over breeze blocks. Are breeze blocks like normal brickwork and simply require wetting before applying the render? If not, how should they be prepared? Also, is 3 sand to 1 cement the best mix to go over all kinds of brick/breeze block surfaces or would anyone suggest a better mix?

Also, other smaller areas (nothing deeper than 3 inches) require filling. Should I use bonding to fill these areas or is cement a better option? I know that bonding is better on low suction surfaces but what constitutes 'low suction' and what constitutes 'high suction'?
Any general rules when and when not to use bonding would also be appreciated.

Thanks in advance.

 

[JerryM]

i've not actually rendered over breeze block so it’s an interesting thought (I know mortar for blockwork is much looser than for brick). all my jobs are on brickwork (damp proofing).

i don't wet brickwork 1st so would say same for breezeblock. I render at 3:1 plastering sand: cement 1st coat and 4:1 second coat (with 2nd applied as soon as 1st can take it)

I would generally fill using either browning or finish on brick (I only use bonding on none brick materials ie mainly wood). depends on depth mainly and size - the finish will crack on a deep fill unless done in a few goes.

I guess if you’ve got render available then render would also be fine.

 

[Richard C]

If you’ve got deep holes, fill with render first & level off but you also need to reinforce any cracks in the blocks or mortar joints with stainless steel mesh at this stage or the cracks will almost certainly come through again. I always render over the mesh but if the wall is generally just loose, crumbly & as you say, a bit of a mess, I would use Bonding as a base out to existing plaster level & skim over the whole lot. Although primarily for low suction backgrounds (e.g. plaster board, steel/plastic capping rtc.), you can still use it on blocks (high suction) & its ideal in this case as it can help consolidate a pretty ropey looking wall. PVA seal the blocks the night before, then PVA x 2 coats immediately before Bonding. It can be applied up to around 11mm thick but on high suction backgrounds, I put it on in 2 layers, the second just as the first is going off; if it starts crazing, damp the surface down with a spray bottle & give it a light trowel as it’s going off.

 

[JerryM]

Richards spot on in the need for the mesh - i too use it in the circumstances he describes.

i like his idea of using the bonding which i had not thought of - i've banked it for future use - thanks RC.

 

[Richard C]

Richards spot on in the need for the mesh - i too use it in the circumstances he describes.

i like his idea of using the bonding which i had not thought of - i've banked it for future use - thanks RC.

 

[DIYDouglas1]

Thats great. Thanks to both of you for your help-much appreciated.