What is 'picking up' and why does it happen?

There are two meanings for this term. One is the process of joining up to 'wet' or 'live' edges on lapping, thinning the finish coat will help keep the wet edge open; the other meaning concerns the lifting or softening of a previous coat of paint when applying a new coat, to do this allow to ry and scrape back. The remedy in both cases is to thoroughly clean the surfaces to remove all dirt, grease and surface contaminants. Then scrape back all areas of poorly adhering or defective coatings to a firm edge, rub down to 'feather' broken edges, dust off and apply a new coating.