Patch Repair and Pointing Mortar

Plastic fibres increase the low tensile strength and are primarily used to control cracking.

NT

They are usually polypropylene filaments, but they can be glass fibres. They distribute shrinkage cracks throughout the mortar, whereas an unreinforced mortar tends to crack more widely in one place.

The fibres keep the shrinkage cracks well below the ~0.1mm width that allows water to penetrate. The result is greater strength and durability.

This sounds like the ideal stuff for filling the gap between my upvc frames and stone sills. The gaps are currently covered by 30mm wide strips that are merely held in place with a white (gone grey) sealant and looks crap.

Is this Pointing Mortar stuff a good choice for this job?

Thanks.

Arthur

In B&Q yesterday, I noticed they're selling bags of fibres to add to cement mixes - supposed to stop cracks appearing, so I guess that's the same thing.

I doubt it. You need something that will accommodate movement, so a flexible sealer would be best. The pointing mortar is not flexible, so will impose huge stresses on the frames or sills, or both, leading to long term damage.

Flexible sealants are not my area of expertise, so I hope someone with more knowledge will be able to advise you.