first off unless you have a keen eye and feel of car body work i would advise not to undertake the filling 1.no offence but you prob wont get nice straight panels 2.filling lets say at home and maybe carting it off to the bodyshop to get primed runs the risk of the filler getting wet,acting like a sponge and releasing moisture through your new paint job in the form of micro blisters 3.as wet sanding is kind of a thing of the past unless you have the proper knowledge of sanding grades,sanders,sanding discs a lot of bodyshops will want to resand your own work again.we get loads of people like yourself and always end up resanding but it also depends on the quality of the job you want.the key as mentioned is good know how (by all means try yourself but ask a good repair shop for guidance)good skills,proper abhrasives/equipment and good products from filler through to final polish.your previous job might of been done with cheap products,cellulose paint, too fast of hardener/thinner in a 2k system causes are endless.when pricing ask will it be painted in a base/clear system or just 2-3 coats of solid 2k paint the base/clear will be more exspensive but nicer.pm me if you need any more info