The purpose of the facecoat without fibres, is to ensure that your finished concrete doesn’t have any fibres showing through.
By pouring a 3mm layer you are putting distance between the face and the fibres. If you cast the fibre filled back mix in too soon or aggressively fibres may find their way to the face.
If the face coat is left for a short while and has time to consolidate then the back coat will not show through.
On larger pours, by the time you’ve cast the face coat, enough time has passed for you to go straight in with the fibre back coat mix.
On smaller pours, a short period of 15-20 minutes should be enough for the facecoat to settle.
On the flip side, you don’t want to leave the facecoat too long because it will start to skin over and cure, which is where you may end up with two distinct layers. When fully cured the facecoat may then peel off.
In extreme circumstances a curing agent can be used if for example the facecoat is left in the mould for extended period prior to the back coat being cast, but this is a more advance method. We recommend planning your project so that pieces can be fully cast in one day, even if that means splitting the project down.