Sorry to say but I think there is no way to correct it! Applying large areas of lime based crepi is quite a tricky job, because the moisture and air temperature are critical and it is these that cause the problems.

A professional will apply a rough undercoat first, allow it to dry, and then apply the final coat. (sometimes three coats are used). The different layers are often applied by a machine so that large areas can be done quickly.

The role of the first coat is, in part, to ensure that the second coat is an even thickness and hence dries at a constant rate.

The warmer the air when the crepi is applied, the faster it dries, and the faster it dries the more the lime is carried to the surface. So crepi applied on a warm day will be lighter than crepi applied on a cool day. Similarly, the moisture content of each batch of crepi applied must be identical.

Joins being visible is, I think, for the same reason – the edges dry at a different rate. Although in any case it is very hard to hide the joins – so really a whole wall needs to be completed in one ‘sitting’.

On walls I have done (never especially large, and only on outbuildings, not important walls) I have also managed to reduce the problem by gently brushing the wall as it dries, including the join. This works reasonably well, but again depends on doing the whole wall in one day.

Another option is to use a ‘wire stippling brush’ (like the macons use)on the wall as it dries – these are available in DIY shops and make the surface evenly ‘rough’, and would reduce the visibility of the joins.

But as for correcting an existing dry crepi wall? I’m afraid I don’t have any great suggestions, sorry!