Tip from a collegue see here:
Rust Removal DIY
Message Title: Re: I'm interested...
Posted by: Tricord on 2003-04-04 at 16:56:35
(posted from: Host: ncgent01.telenet-ops.be IP: 213.224.83.70)
Message:
Well Yeddie, for major rust holes, you will need to cut out a piece of metal and weld another back in, after which you sandpaper or smoothen out the welding ridges in order to restore the body invisibly. This however, requires sharp welding skills (it must be a strong joint and yet it mustn't be too large either, otherwise the ridge will remain seeable).
Seeing as I don't want to start welding yet for various reasons (cost being one, and lack of skill another), and that the rust holes are fairly small (max 1-2cm in diameter) you can use special types of filler paste to fix the car.
The procedure is as follows: for really rough areas, mount an iron wire brush on a drill and remove the big rust as such. Once the area smoothened out, you need to sandpaper it further until you get healty, blank metal (if this means to enlarge a small hole, then so be it. you can't leave the rust in). Also, remove the paint in a circle of 2-3cm away from the rust spot, to allow the reparation to go over seamlessly in the remaining paintcoat. For surface reparations with minor rust without a hole, you can use standard polyester filler paste (check if it's suitable for metal. Mine is also suitable for concrete and other materials). Surface rust will typically cause little dents in the metal, which are areas where rust took more effect than others. The paste will fill these holes and will harden in about 30min. After that, you must sandpaper all excess of dried paste, with or without water, and if neccessary, re-apply a new coat of paste and sandpaper again. Repeat as neccessary, until you obtain the original shape smooth as silk. You will then need to clean the area with water to make sure all dust is gone (because with sandpapering the paste, there will be dust :)). Then you need to apply primer, which is the base protective coating. When the primer is dry, you can paint over it (in multiple layers!). If your car has a 'metallic' color, you will need to add a layer or two of transparant finish coating, to give the metallic aspect to the color.
If you are repairing a larger hole, you will need filler paste for use with fiberglass sheets, both of which you should be able to buy in a car shop. If you want to use the standard filler paste you need that so-called chicken wire. It's a small-pattern fence you use to surround a chicken area :) Bend it inside and around the hole so that it has roughly the same structure as the metal that used to be there, but that rusted away. Then reconstruct the metal with paste, using the piece of fence as support. Then proceed with the rest as above.
This is a good way to cheaply 'fix' the rust problems on your car, however do not use this method for structural repairs. You can only repair cosmetic rust, you shouldn't expect the hardened filler paste to be as strong as metal in areas that are under stress.
If you go the welding way, you might as well completely dismantle your body and do it right from the ground up, and repaint the entire body in a painting cabin. But that will cost you a huge deal more than this method. 30EUR should get you started with all the products you need, including the paint and the transparent finish. You can find everything in spraycans. I was lucky enough to find my car's color - Ascotgrau - readily available in a spraycan at the car shop.
Good luck, let me know if you have further questions.