Patio jointing compound for narrow gaps.

Hi all,

We Put down a new patio last year, It was with patio blocks that were approx 60-80mm thick, 120 wide and 500 long. (off the top of my head)

They were put down as tight together as possible, but still small gaps, maybe 3mm.
Is there a brush in compound that I can get that will work with tight gaps?

Rompox is only recommended for 5mm Plus.

Thanks