Here are a few ghost signs which I see regularly, because they're alongside the New Cross Road. The first one is practically opposite my street, and is a nice example of an advertisment for Bryant and May matches. It gives a strong 'Buy British' message: these are 'British matches for British homes'. The order at the bottom to 'save the panels' presumably suggests some kind of promotional gift scheme.
The second sign also refers to matches - Criterion, this time (see a clearer example on the Ghost Signs blog). The text and an image of the matchbox survive in a greyish colour, the bottom layer of a palimpsest. Overlaid are advertisments for 'Redfern's' (I've no idea what the product was) and for Nestle Milk, 'richest in cream'.
Clutten, Swan & Co, Artistic Ticket Writers have painted their publicity over one for sanitary engineers. Palimpsests generally show where a wall was rented for a fixed period, after which a new 'tenant's' advertisement would appear. However, given the trade of the newer advertisers, it's fun to imagine that they were maybe carrying out a 'repossession' for non-payment!