Scale insects on Citrus Trees

I have two lemon trees which I've had for about 14 years that grow well and fruit. I place them outdoors in the summer months and bring them into my unheated conservatory in the Winter. For the past few years I have had a real problem with scale insects. I try scraping and washing them off, but they keep reappearing. Now that the trees are larger it is very time consuming inspecting every leaf about once a month. The scales seem to be worst in really awkward places in the leaf junctions and the thorns are always where you try to get at the scales!! Is there a good way of removing or killing the scales without having to resort to cleaning each individual leaf! Suggestions welcomed.