After you have the car stripped, the metal work should start next. Metal work means fixing the small damages that he body of the car might have sustained. This is important because paint cannot hide the dents and scratched in a car. Small dents can be worked out by using a hammer and dolly or a semi pointed probe worked across the metal on the back side. I worked out several small dents this way and did not have to use any filler at all in those places. Larger dents have to be banged out using the hammer and dolly the best as can be done. Unless you are an artisan then you will have to use some filler on large dents. Be sure to not have any high places in the metal or you will have a bump in the final paint finish that cannot be sanded down. That way, you can prepare your car properly before you go on painting your car
Often, high areas can be "shrunk" by using a pick hammer with a dolly in the back side. What you must do is to exchange the large dent or bump for a whole bunch of small bumps that are easier to fill and cover. Lightly just tap over that rise area working from the edges to the center and the bump will slowly disappears. This may take some practice to get good at. If you drill holes to pull out a dent, then you must weld up the holes when done to prevent moisture from getting to the back side of the filler material through the holes.
There is only one way to fix rust. Cut it out, remove it and weld in new metal. Completely remove all rusty metal from the car, don't cover any up or the rust will grow. I use a Plasma cutter to cut the metal but quality tin-snips or a nibbler will work just fine. If your car has some surface rust than this is OK as the paint will completely seal it and prevent it from growing further. Cut a piece if 18 gauge metal to cover your rusted area that is about 1/4 inch larger than the hole. This metal should be plain steel and must not have any coating (e.g. galvanized coat). The base metal should also be very clean for at least an inch away from the weld. Use wire brush or sander to prep the metal.
As an option, you can add a depression to the body metal for the patch to set in. A tool is made to do this but. This tool only makes 3/4 inch long ridges at one time and can cause some warping of the metal around the patch area. The new metal must be completely welded in using a good MIG (metal inert glass) welder. Low current 110 welders just don't do the job with the exception of the Lincoln SP100 or SP125. I use a 200 Amp Century. Turn the welder up to a voltage that would normally melt holes in the metal if welded continuously. Then tack the metal in place using short bursts of weld (1 second) about every 4 inches around the patch.
NEVER try to push the metal in place while welding. The metal should fit the contour of the body naturally with only a very slight pressure needed to hold the pieces together. If you press hard while welding then you add stresses to the patch and will cause warping in the welding process. Once the piece is in place you can start welding it solid. This is very important to do slowly. Weld in about 1/4 to 1/2 inch strips around the patch with one hand on the surrounding metal about 3 inches away. Each weld spot should spaced apart around the patch area. You keep doing this until you fill all the gap in. If the surrounding metal gets too hot to hold your hand on then quit till you can lay your hand over the area that is being welded.
This process may take 20 minutes for a 6-by-6 patch area. If you rush the welding then the metal gets too hot and warps. Sheet metal under about 12 gauge is made by a cold rolling process. This process puts stress on the metal released in the welding process which ca cause warping. Keeping the head down reduces this possibility. The welding should be continuous and not just spotted. Moisture can come in from the back side and cause rust to grow under the filler material and cause the filler to lift off. Grind the weld lightly when done to reduce any high spots in the weld. Before applying filler, rough up the metal with a 24 grit disk on a 7 inch grinder so the filler has something to stick to. You may also want to use the pick hammer and slightly dent the metal in around the weld area so that less filler is needed to cover up the patch.
Patching Up Small Holes
For surface rust in areas like the floor board where dents have gotten deep or there are some Swiss cheese-type holes less than the size of a pencil, an alternate to welding can be done. Use a wire brush to remove all loose scale and sand away paint from the surrounding area for about 6 inches. Clean up all dust with a vacuum. Apply POR15 metal prep to the area to etch the new metal that is not rusted. POR15 will not stick to clean metal unless chemically etched, but it sticks very well to rust. Apply a heavy coat of POR15 paint over the entire area. Lay a layer of fiberglass cloth over the rusted area and coat with another heavy coat of POR15. After the paint is dry to the touch, add another coat and a layer of slightly smaller fiberglass cloth. After it has dried to the touch then add 2 more coats of POR15 alone waiting till the paint has dried to the touch between each coat. POR15 is an amazing paint. It does not ever fully get hard. It dries faster when it is damp out. It will not lift with the formation of rust like normal paint will, and is so strong that you will NEVER be able to peel that fiberglass cloth off again.
The patch will be nearly as strong as the original metal. The stuff is not really paint, but they call it paint for lack of a better word. You can also use it for areas of heavy rust on the body that has not gone completely through. Simply paint a couple of coats over rusted metal before filling and painting. If you are going to top coat the POR15 then you need to spray a light coat of primer on it before it completely dries or sand it so the top coats will stick. You don't need the fiberglass cloth on an area that does not have holes through. I also recommend using it under fenders where rust is on the back side and over the back side of the welded-in patches.
POR15 is not the same as rust converters (e.g. "Extend"). It does not convert the rust, but instead covers it up which helps in preventing moisture from getting to the rust which stops any further rust spread. Don’t forget to wear plastic gloves. POR15 sticks to skin like crazy glue. If it dries on your hands you will wear it for about 2 weeks. (I tried Acetone, Xylene, Toluene, Methel-ethel keytone, Tricholrethene, gasoline, Alcohol, Lacquer thinner, Carb cleaner, Brake cleaner with Hexane, sand paper, and soap to get it off. Nothing worked.)