The Art of Stripping (we know what you're thinking - wrong site)
With a few exceptions, if you have an existing finish you will need to remove it to get a first rate job. The exceptions would be if you already have had this done and there are only one or two (max) paint jobs on sound bodywork with no cracking, blistering or delaminating (peeling) of the current finish.

If you are not sure how much is there, there are magnetic gauges available for steel surfaces that measure film build. The cheap gauges (such as available from Marhyde and Eastwood) are very inaccurate and you usually have to deduct about 5mil from what they read (try them on bare steel first and you will see what I mean). The paint companies use an electronic type (they run around 2Gs) and they will tell you if you have too much, the accepted norm is about 8 mils maximum to refinish. A normal base/clear paint will total 2.5 mils + approx 2 mil in primer. It is recommended to strip if you have a "soft" finish, such as enamel or lacquer as the solvents in current paints will "heat" them up - to test take a rag soaked in the reducer of the paint you are going to use and let it sit on the surface for a couple of hours - it should have no effect on the finish - if it does -strip it!

There are a lot of ways to strip paint but one of the most important things is how you treat the metal and how soon after. Bare steel left unprotected starts rusting right away from humidity in the air. It is important to get a good epoxy or etch primer on the bare surface - the sooner, the better. Do not put a regular lacquer type primer (ever) or even a two-part urethane or polyester primer unless it is specifically recommend for totally bare substrates (few are). When cars are painted at the factory, they go through several acid baths to thoroughly clean the metal before a cathodic E Prime electrostatic dipping to protect the metal (prior to primer). If you have left your metal for more than a few hours unprotected it is a good idea to scrub the surface with an automotive metal conditioner (a mild acid - wear gloves) and a red or grey scotchbrite pad, and dry, just before you prime.

The methods for stripping paint are varied, from chemical to abrasive, they all have certain advantages and disadvantages. Chemical stripping can be done by, dipping in an acid tank (excellent cleaning), or by hand using paint stripper. If you are going to strip it yourself make sure you wear suitable gloves, a painter mask, old clothes and have some sort of ventilation. Paint strippers attack oil based products, so double mask areas such as rubbers and spread some newspapers on the floor. Paint stripper works best on warm surfaces and apply with a cheap paintbrush - brushing one direction only (back brushing stops it from working as well). After it has stopped bubbling start to scrape it off with a flexible putty scraper, you will probably have to apply and scrape a few times. Water neutralizes the stripper and can be used as a final wipe, but this also starts rust, I like to use lacquer thinners for a final wipe with paper towels. On stubborn areas (edges) take fine steel wool, a red scotchbrite pad or sand paper to get them clean.

Media blasting offers a lot of options today. Commercial sandblasting is fine on thick metal pieces such as frames, rear ends etc,; but can warp hoods and door panels severely. If you use fresh fine sand and watch your pressure, you can strip fenders and other sensitive sheet metal parts. The sand gets everywhere so be warned. There are lots of different blast mediums that are gentler such as soda, crushed acorns, plastic pellets, etc., talk to a specialist (and friends) to see if this is the route for you. The other option is using 80 grit on a DA sander, effective, but be careful as you can damage raised bodylines in seconds. There is no one best way - just study a few dancers (just kidding).

John