For stock cars and bangers allow around £30 per item/sponsor. Some items are very simple to do (race numbers) and others can take much longer (lengthy business names) but if you estimate £30 per item you should be in the right sort of ballpark. This price guide assumes that there will be an area for me to paint 'Big White Frog SIGNS' somewhere.
This is a guide and not a guarantee. Logos, tight fonts and long business names could push the average item cost to well above £30 each but I'll do my best to keep the average price as near to £30 as possible. Smooth panels will speed up the outlining process but rough panels will slow it down. For cheaper work, signwriting on rough bodywork can be outlined with an airbrush rather than with a signwriting brush but the result will be fuzzier than traditional methods.
IMPORTANT: For best results keep things simple. For instance 'JACK SMITH Builders' and maybe a phone number. People will Google the business to find web addresses, emails and other info. Too much info will destroy the look of the car.
