How can I tell if an old coat of paint is latex or alkyd based?
Simply dip a cloth in rubbing or wood alcohol and lightly rub the surface. If the rubbed surface is not altered and the cloth is not stained, your surface is covered with an alkyd-based paint. Conversely, if the surface is altered and the cloth is stained, your surface is covered with a latex-based paint.
On a white surface, use a dark-coloured cloth. On a coloured surface, use a white cloth.