to remove the old paint Id sugest u use paint stripper. Simply remove or mask up all crome and plastic trims scuff the surface up first with a bit of 80 grit till there is no shine then apply the stripper and cover it with some plastic and wait till the paint starts to lift depending on the number of coats of old paint you may need to repeat this. once all of the old paint is gone get some old rags and some thinners and wipe the panel clean then orbital sand with 80 grit to remove any remaining litte bits of old paint. Using grinders with 36 grit tends to scar the metal and if you dont know what you are doing heat it up causing panel damage. Sand blasting will generally provide disasturous results on exteriour panels but is excellent for engine bays and door jambs I have been through 3 sandblasters and I am yet to see a car that has been sandblasted that isnt fucked as a result, the people who do this type of work seem to be heavy handed morons who dont give a shit about light gauge panels. Coat all panels with a good quality etch primer and dont leave out in the weather. The panels will be then ready to be Panel beaten. If you are going to be bothered to do this dont waist your time, paint it in a good quality 2 pack and itll last for ages