To add a "little" more to what the other guys said......
“Basics of Basics” Door skins
By Brian Martin
First off, to remove the old skin, take a grinder and grind the edge door where the skin folds around onto the door shell. Grind until you see the three layers of metal. Don’t worry if you grind a little too much, if the shell gets ground a little it is no big deal. On the spot welds you can drill them or grind them, sometimes both. By grind I mean a die grinder with a cut off wheel, just set the spinning wheel on the weld moving it back and forth till you have cut though eliminating the weld.
To prepare the new skin, take a DA or similar tool and sand the OUTER edge of fold in the new skin a little. I am talking about the edge as it will be folded onto your door shell. If you LIGHTLY sand this edge the lip will fold MUCH easier when you install the skin.
If you don’t plan on bonding the skin on, I highly recommend it. It is a corrosion fighter like none other. With the door sitting on it's back (the interior side down) clean the edge where the panel will bond with an abrasive disc to bare metal. Don't use a grinder, it removes metal. Then you will put a small ribbon of panel adhesive, NOT door skin adhesive but the Panel adhesive, it has a higher strength and longer working time. Spread out the adhesive with a plastic spreader so ALL the bare metal is covered. Then apply another thin ribbon on the inside edge of shell where it folds down towards the interior.
I don't know what primer you will find on the new skin, if you can be assured it is good quality and has bonded well, leave it. If it is questionable, then sand it out and apply a good epoxy primer.
After you have a good primer (or left alone) you need to simply scuff it with a red scuff pad down in the area that will be bonding. And believe me DON'T over do it with the bonding! It WILL go around the other side of the fold without even trying and get on your dolly and hammer!
Lay the skin down on the door and position it. Clamp it down at the top where you won't mess up the outer skin. I used a rubber dolly, if you don't have one take a flat dolly and tape a rag or something on it to provide a little cush. A neat home made dolly for this can be fashioned from a 5" long piece of 2x4 hard wood and a 3M rubber squeege glued to it. Hold the dolly on the top surface right over where you are going to fold the lip, strike up on the fold with your hammer while pushing down on the dolly, strike it at an angle so the metal "wants" to hold and NOT lift the skin off the door.
THE TRICK!...... Don't fold it very much at a time, I mean VERY LITTLE, about an 1/8" MAX, maybe even 1/16". Go around the WHOLE door before folding more. Go around and around until you have it folded down almost flat, about 1/16" from touching.
Turn the door over, being particularly careful not to rest the door in a way that will bend your new skin! Now strike down with the hammer while you are still supporting with the dolly to close up the fold a bit more.
ANOTHER TIP!....DON'T smash it down!!!! Two reasons, one you will distort the outside and two you need to leave the bonding in there for it to work.
STILL ANOTHER TIP!.... Use a number of hammers that closely match the shape of the door, if you are in the rounded area at the top of the door, use a domed hammer.
After all folding is done, fine tune the edge so that there are no high or low spots. You can run a vexin file over these areas to spot highs and lows and cut a LITTLE if you need off of high spots. Then using a DA on grinder mode with a 120 disc you "block" the panel around the edge to perfection.
You can spray a weld thru primer on a couple of inches or so at the corners so you can then weld there and not burn the adhesive. But be ready to trial fit the door and twist it if need be for your weld it. You should always trial fit the door before the adhesive cures and “massage” the door to fit well then weld the corners on the back side so the door can’t twist while the adhesive cures.
If you have any fears of not being able to get the door skin on and folded in the working time of the adhesive, don’t use it! All you need is a little tack weld at each corner on the inside at the folded lip. Just go ahead and prime all the hidden areas and after the skin is on, put a seam sealer on the folded seam and spray a cavity wax or underseal around the seam from the inside.
Read the recommendations on the adhesive you are using, some want the metal bare while other want it to be primed, read the tech sheets.