By Del Sessions and with thanks to Ralph Maines
There is a lower door rust repair that you can do at home with a minimum of tools, you don't even need a mig welder or an air compressor. Duramix makes a two-part urethane liquid bonding adhesive for bonding door skins onto door frames. The adhesive is laid on with a special gun similar to a caulking gun; the gun has two cylinders and the adhesive is mixed together as it goes through the mixing tip on the end of the gun. Purchase special gun where you buy the adhesive.
Duramix adhesive has been on the market for about four years. Duramix put both rear quarter panels on a car and crash tested the rear. Judging from the photos, it looked like the car was totaled but the panels remained securely attached to the car.
Duramix has three adhesives: small panel, medium panel, and large panel. The medium will give approximately 45 minutes work time, which is way more time than you will need to put everything back together.
Another company that makes a similar product "The Fuser" bonded on an entire roof panel and rolled a car and the adhesive held up. Both products are very good, but I don't think I can convince myself to do that without a few welds to make it safer.
About four years ago I used the adhesive to install both complete outer door skins on a 911. This same car has been crashed twice since then with damage to both doors but no failure of the adhesive. We repaired the doors with no problems. I personally have a '62 Oslo Blue Porsche 356 coupe that is a very good driver but certainly not a concours car. The left door started showing signs of rust at the bottom outer panel, so I decided to give the Duramix urethane adhesive a try and splice the lower 3 inches of the outer door skin to the door bottom. (I'm sure you're not going to do this to a Carrera II Cab.!). If you think about it, it might even be a better repair than welding and the company claims it doesn't absorb water. But if you have a Carrera Cab, you probably would not be doing the rust repair yourself anyway.
The first thing before starting the repair is to check the door gap and fit. I don't recommend removing the door shims to improve the fit, do this only if you have had experience or if the door is horribly out of whack. If the door gap at the rocker panel is a little large, you can make the new piece of metal a little larger. Remove the two door pins (not the bolts) and take the door off. Make a template from the lower door, or lay the new metal over the door and mark it from the back side of the door, probably about 3 1/2 to 4 inches wide - to get above the rust. Make the new panel about 1/2 inch wider than you need because you will slip it up under the door panel where you have cut it off. I use an air tool with a cut-off wheel to trim off the old door panel. If you don't have an air compressor, you can buy a small hand-held electric grinder; there are cut-off wheels made to fit these. You will probably need about 5 cut-off wheels for a project this size. Mark the upper part where you plan to cut. Be sure to wear a pair of tight fitting goggles or a face shield as well as gloves. The sparks really sting! Pay attention to where the sparks are going, as you can easily start a fire. Make sure to move you wife's new BMW out of the garage, sparks will damage the paint and glass. If the sparks should hit near the gas filler, you might not be around to explain what happened! When you make the cut, be careful at each end of the door or you will cut into the door framework.
Grind the bottom edge of the outside door skin until you reach the inner panel. At that point, there may be a few spot welds still holding. Grind them a little more and move the cut off panel back and forth until they separate.
Now clean up the rust inside the door. It's very important to get all the rust off, especially where the panels will bond together. It can be removed with a wire brush on a drill, but it takes an awful lot of time. If the rust is bad, take it somewhere and have it blasted. Be sure to remove the glass, regulator, and glass frame first. Fit the new panel to the door. I used a flange tool to offset the top edge of the new panel. You don't have to do this, but it makes a neater job. Slide the new panel up under the old door skin and when you get it in the position where you want it, scribe vertical marks on both panels and a horizontal mark on the new panel where it lines up with the old door skin so you can easily find the correct position again when you are bonding it together.
Grind the area where the adhesive will be applied with a 16 grit disc or you can rough it up with the cut-off wheel to give the adhesive a better bite to the metal.
Get everything set up and fit before bonding. Lay an even bead with the mixing tip. The amount seems less critical than evenness end-to-end If there are uneven amounts the seam may show slightly in extreme heat (see "afterwards", below).You may have to put a piece of wood inside the door to keep the new panel from dropping and you may have to put some weights on the old panel to press against the new panel in the spliced area. Also put clamps or vice grips along the bottom with very little tension so as not to squeeze out too much of the adhesive. The adhesive will work just fine for a door bottom (door frame). You will need a few small screws on the inside of the door (where the trim panel will cover) to hold together. After the adhesive dries, remove the screws and fill the holes with additional fresh adhesive. Clean off almost all of the urethane on the outside of the splice. Leave the tip on the gun when finished. There will be plenty of urethane left; just put on a new tip and you're all set to go for your next project.
(I did not fold the new outer panel back over the door bottom. After the adhesive dried, I laid a few pieces of masking tape, one on top of the other, about 1/4 inch from the bottom on the back side and applied a little adhesive and pulled the tape off before the adhesive dried. I later sanded it down a little and it looked just like the factory fold but with no chance of water collecting there).
Important: After the urethane has cured, mount the door back on the car and check the fit. I thought I had the new piece placed perfectly on the door but when I put it back on for a check and tried to close the door, it hit at the lower front corner where the lower front fender and rocker are welded together and it's usually a little thicker so there's not as much clearance there as at the rest of the fender. I had to remove the door and grind a little off the front edge of the new door panel. Also I tapped the lower edge in or out to line up with the rocker panel. This tapping won't crack the urethane. After body filler, mount the door back on the car and check the fit before priming. This door took me 6 1/2 hours to get to the plastic filler stage compared to 18 hours if I had used the welding method, so you can see that this product is definitely a time saver. It took me 3 1/2 hours to finish this straight door off with plastic filler. A rough door could take up to 12 hours, and you may want to have a body shop do the finish work.
Wearing goggles and gloves, brush POR-15 or Wurth Rust Guard on the inside of the doors. These are black paints that dry very hard and hold back the rust/moisture better than anything else I have found. Warning: if these products dry on your skin, you can't remove them and it takes about a week to wear off! Later, under-seal the inside of the door.
My main concern before splicing the door was not the strength, but rather the expansion and contraction of the two materials at the spliced area in hot weather. For example, sometimes I see a line where fiberglass has been bonded to metal. After priming the door and laying it flat on a bench for about 3 hot days in the sun, a faint line of about 6 inches long appeared at the spliced area, rear only. When I blocked the primer, it came out very quickly. I never laid it out flat again in the hot sun. It gets much hotter from the sun that way than if it were upright on the car. After painting and mounting the door on the car, we had a record 108 degrees in San Jose. I was not able to see the line again. I would not recommend installing a nose panel this way. The upper panels on the car get much hotter than the lower door area and there is a good chance of having a line. It would take additional tests to see if more or less adhesive would change this effect.
I know what you're thinking now... battery floor, floor panels, etc. If your battery floor is bad, this would be an easy repair and a good place to start and learn how to use the cutter and urethane adhesive. Before you start cutting out the battery floor, be sure to cover the gas tank so that sparks cannot reach it. Also cover all vents from the tank (there is a gas tank vent that leads to the underside of the car near the gas tank). On a 356 T6 body, cover the fuel shut off valve under the tank on the underside of the car. They often sweat and leak a little. Make sure no sparks can get to this area.
If you use this type of repair on battery floors, the ground strap mount will have to be moved to the wall behind the battery. After bonding, another option is to have the welding shop put a couple of tack welds with the mig welder at the rear of the floor near the battery ground. This should only take a couple of minutes.
I feel this urethane method would work just fine on floor panels, but, after the bonding process, I would urge you to take the car to the welding shop and have two or three tack welds done with the mig welder in each corner as well as three or four tacks along each side, simply because the floor is a structural part of the car. This would take the welder about 15 minutes to complete. There are articles in the Registry on floor replacement. Check them out before starting this project.
Remember that with this repair as well as with the doors it is extremely important that the matching surfaces be clean and rust free before applying the urethane. Don't even think about mounting the jack receivers with the urethane, have them welded on. There is very little area for the urethane to stick to.