SBR primer going patchy on old screed

[jayc89]

I'm priming a screeded floor ready for a self-levelling compound. I've applied it as per the SBR instructions (3:1) but it's going really patching. The screed looks incredibly porous, the primer is disappearing within seconds and only patches of the screed are remaining white.

 

Is this normal? Should I apply more/stronger coats of the SBR mix before the self-leveller? 

[PeterW]

Get an old mop and bucket and lather it on, then wet the floor til it’s sopping and then lay your SLC.
 

Depending on the manufacturer you may need an extra 1/2 litre of water per bag, mix with a whisk and let it stand for 5 minutes, mix again then pour. Squeegee to move it and a spiked roller to get the bubbles out and flat it off. 
 

@Nickfromwales anything to add ..?

[jayc89]

Thanks @PeterW. After 4 coats with a paint roller it was looking much better.

I’ve only gone and run out of SLC now!

[Nickfromwales]

2 hours ago, PeterW said:

@Nickfromwales anything to add ..?

Only that this is exactly what to see / expect  

The slab is porous, and will suck the diluted primer like Gillian Taylforth, so nothing to worry about there, just keep lathering it on until the floor gets as saturated as you can get it.

I always get the SLC down over the floor whilst the diluted primer is still wet ( so not technically cured / left to dry ) which I find leaves the SLC to flow far more readily. Also, if it's a 2-part SLC with the 5L of addmix, I always throw a 1/4 pint of water in to the mix immediately before pouring.

 

2 hours ago, jayc89 said:

Thanks @PeterW. After 4 coats with a paint roller it was looking much better.

I’ve only gone and run out of SLC now!

And whatever you do, don't run out of SLC 🤣 

[jayc89]

Thanks @Nickfromwales. I used an online calc to try ensure I had enough for what I needed + 10% but it hasn't gone anywhere near as far, I probably need another full bag, which I'll do today. (Mapei 3240)

Should I sand back the section that hasn't been done yet, but was SBR'd yesterday, and reapply that first?