Depends how wide the cracks are to start with. If they are hairline cracks use fine surface filler rubbed in with a sponge. If they are slioghtly larger use normal filler. If they are 3 mm or more yes, cut them out and use plaster. We don't recommend different manufauctuirers for paint but the paint you can find in our toolstopre is perfectly good. See also our project on repairing cracks.
thanks for the reply,it has solved another problem i have,but i must apologise for not making myself clear,i forgot to mention that my problem is external,in a rendered gable end. the cracks are only a couple of mm. wide. one about 3m long and the other about 3.5m long.
Thanks for this information - we have recently finished plastering and have discovered hairline cracks along many of the joints. Can you recommend a fine surface filler? We believe the problem has something to do with expansion and contraction due to extreme dry weather conditions, so would be interested in a filler that will be flexible enough to prevent further problems.