Synthaprufing a wall

[RogerS]

I want to apply two coats of Synthaprufe to one of our internal (non-DPC'd) walls.

The instructions say that you need to finish with 12mm of plaster and so presumably I need to chisel out the old plaster to approximately that depth...so that the finished plaster will be more or less in line with the plaster higher up the wall?

If so do I need to do any preparatory work to the chisselled out wall prior to the application of Synthaprufe? The instructions are silent on this issue. The current wall surface is very friable and powdery (efflorescence?) and my concern is that if I do chisel out the plaster then the surface left will be too friable for the Synthaprufe to adhere to.

In some areas the finish top coat is no longer adhering to the underlying bonding. Part of me wants to just hack it all back to the (probably) rotten oak sole plates but don't want to make unnecessary work for myself.

Last question. The instructions refer to blinding the final coat with sharp sand. 'Blinding'...does this mean throwing sand onto the still-wet Synthaprufe to provide a key for subsequent plastering?

What would you guys do?

 

[jasonB]

I would take off all the plaster as any that is left behind the synthaproof will be subject to moisture. Give the wall a good wirebrushing to remove as much loose material as possible before coating.

Blinding means as you say throwing dry coarse sharp sand at the wall to give a key. I would be tempted to render and set rather than use bonding and finish.

PS don't use your best paint brush to apply the synthaproof :wink:

Jason

 

[RogerS]

to render and set
Jason

Thanks Jason.

Render and set? Cement render I think I understand but 'set'?

Many thanks

Roger

 

[jasonB]

Coat of finish plaster on the render scratch coat.

Jason

 

[RogerS]

Thanks, Jason...

 

[RogerS]

OK - hacked off the plaster to reveal, as suspected, very soggy sole plate. Now I'm not going to replace that but I'll chop off all the rotten oak.

Usually I would then liberally pour a load of Cuprinol anti-wet rot stuff all over it but would this affect the Synthaprufe?

Many thanks

Roger

 

[jasonB]

Give the 5star wood treatment a week to allow any solvents to evaporate and you should be OK.

I would be worried about treating a wall with wood in it in the way you suggest, by covering it in synthaproof you will be sealing in any moisture that is traveling up from the ground. Better to go for a breathable finish or insert a DPC below the timber sole plate. I had assumed you were talking about a masonary wall in my earlier replys.

Jason

 

[RogerS]

Well the sole plate has been there for about 300 years.

In an ideal world I would excavate footings, insert a DPC, replace with a new sole plate and carry on from there. Somehow I don't think that I'll be going down that route....otherwise we'd end up doing the rest of the place in the same way and then when would I get time for woodworking!

I know it's not ideal to seal in the moisture but it's not particularly load bearing where it's rotten-ish. It's an internal wall and I like the idea of breather paper..even to the extent of leaving the other side exposed (as it goes into the utility area) and so allowing the sole plate to dry out (maybe..be a long time I reckon) but I reckon the smell of damp rotten wood will be too offputting.