Yeah bondo is NOT for rust repair as another stated. it is for filling minor surface imperfections such as small dents etc. on already good sheet metal. bondo can be used as the last stage of body work, after the rust has been repaired and the repair is ground down flush the best possible.
I have done a decent amount of rust repair on my own cars and others and here is my process: i;m just gonna wing it
- remove all surrounding trim
- clean the area with wax and grease remover (or another solvent)
- mask off anything outside of the repair area with thick layers of tape and paper, to prevent accidental damage from a slipped tool or something, or from sparks etc.
- cut ALL of the rust out, or as much as possible. the goal here is to be left with only clean sheet metal to work with. any rust left will spread and come through the paint again. do it the best you can so your repair will last as long as possible.
- grind off the paint and coating surrounding the cut out hole you just made. there should be a "border" of clean exposed freshly-ground sheet metal around the hole, maybe an inch or more thick.
- at this point you should have a hole in the metal that you just cut out where the rust was, no rust left in the vicinity, and a fresh ground border around the hole. at this point I would clean the entire area again with the wax and grease remover. and re-mask if you damaged any of the paint. For the next part, which is filling the area, there are several methods. I personally like to use fiberglass cloth in stead of welding in new sheet metal, because fiberglass does not rust, is easier to sand, and the resin will drip to the edges of the fresh ground surface, "sealing" any exposed sheet metal. but fiberglass is messy and can take getting used to to get your right shape. let me know if you need some instructions on how to repair an area using fiberglass cloth and resin. it is my prefferred method :)
- so, if you are doing fiberglass, ask me and i will give you some tips and instruction on that. if you are using sheet metal, the easiest way would be to get a pre-cut section from a donor car as mentioned above. it will lower the amount of body work to be done. if you are using new sheet metal, you will need to cut and bend to approximate shape, but a smidgen bigger, so the edges of the new metal patch will overlap the edges of the hole. place the new metal over the hole, and tack weld it in place. inspect the new piece and if it is positioned how you want it, weld it in place. Don't use a continuous weld around the outside of the patch. weld in small sections and leave breaks in the weld in the case that the weld cracks, the crack will stop at the break.
- now that the patch is welded (or fiberglassed) in place, take a grinder and grind the edges of the fiberglass, or the welds down to be almost flush with the surrounding sheet metal. this is where good welds come into play because if your weld did not penetrate good, it will be easier to grind off the patch. the purpose here is to just try to get rid of the ugly protruding welds or edges, to make the patch sort of roughly "blend in" to the surrounding sheet metal. it does not need to be perfect, we will worry about that afterwards in another step.
- this would be a good time to clean again with wax and grease remover and clean rags. blow the area with a blow gun.
- now that you have the patch edges ground down and very roughly "blended" with the surrounding metal, it is time for the filler work, or if you are super ambitious you can try to do old fashioned metal work. lol.
- before you stary applying filler, take some medium sandpaper and scuff up the old paint and clearcoat surrounding the bare sheet metal. the point here is to again blend, but this time you are blending the old painted surface to the rough ground sheet metal. you don't want to put body filler over top of un-scuffed or un-sanded paint, it will most likely crack off/peel off. the rough sanded paint surface gives the filler something to grab on to. and make sure to blend the edge of the paint to the sheet metal; it will make sanding the filler easier. the sanding here and later when sanding the filler is time consuming, but do a good job. good prep is key.
- For the first filler step, I like to use short-strand fiberglass reinforced filler. It is body filler with chopped up fiberglass mat mixed into it. It is heavier/thicker then "bondo" and has some structural integrity to it thanks to the added fiberglass strands and different makeup, as to not crack. It can go on thicker and fill small gaps and holes without posing a problem. "bondo," or other brand lightweight body filler is only supposed to be for thin surface layers. and may crack and stuff if put on too thick. it is more brittle. so, take the fiberglass filler, and spread a meduim-thin layer over the patch repair area, and a little bit around it. when applying any filler, try to keep the edges "blended" and the entire application even, as it will make sanding easier.
- wait for the fiberglass filler to dry. Block sand it BY HAND with rough to medium grit (maybe around 80) sandpaper using good sanding technique (ask me if you want more info :) ) until you get an acceptable surface. you may have to re-apply filler and re-sand several times. the better you get, the less times you will have to re-do. between each filler application, re-clean and blow off the area. the fiberglass filler surface dows not have to be perfect, I only use it to fill the more rough areas such as the welds and patch edges, and to get the shape of the surface blended in with the shape of the surrounding metal. it is ok to use this fiberglass filler a little on the thicker side. (read on back of container to see thickness)
- after the fiberglass reinforced filler has made the shape of the repair area mostly match the shape of the surrounding area, it is time for lightweight body filler, (called bondo by many, though the particular "bondo" brand of this stuff sucks IMO. get another brand.) the purpose of the lightweight body filler is to really make that surface smooth and free of most imperfections. remember to at least blow off dust between filler layers. apply this filler in a thin, even layer covering the entire repair area. remember to stay on the scuffed/sanded surface. you will block sand it agian by hand only, no power tools with a meduim grit sandpaper, (80 grit or preferably finer) until the surface is nice and smooth, and even. this can be your final filler layer. this is pretty much what the final product will look like (minus sanding scratches). so feel it with your hand flat on the surface and feel for any bumps, wiggles, low spots, etc. and use this filler until it's smooth and even :) around the edges i sometimes use my fingers without a block, or a DA sander to get some of the tough edges to blend, but most filler sanding should be done with a hand sanding block.
- sometimes I use what is called "icing" over top of the lightweight body filler. it is even lighter and thinner then lightweight body filler. it is used to fill gouges, pinholes, rough sanding scratches, etc. just to get the tiny imperfections out. using this will make the next step a bit easier.
- next step is priming. I like to use what is called "2k primer" because of it's ability to fill small surface imperfections such as sanding marks, and it's 2-part catalyzing formula cures tougher and faster then one part primers. you don't need to prime surfaces that still have old paint on them. just the filler/metal. the primer's main job is to provide an acceptable surface for the paint to adhere to. so, before you spray, blow off the area to get the dust off. mask off the area that you don't want primed. clean the surface with wax & grease remover. spray the primer.
- now wet sand the primer by hand with a wet sanding block and 400-600 grit wetsanding paper to get the edges of the primer to blend in, and to even the primer to a smooth-as-can-be surface. you may be able to use as rough as 300 grit paper or lower. but look at your paint can instructions first. if you sand through the primer you will want to re-prime and re-sand until you get a perfect surface, free of pinholes, scratches, etc. as stated before, sanding is time consuming. be patient with it. do it good, this is how the final product will look. any imperfections present in the primer will be magnified by the clearcoat. you wnt to get this as good as you can get it.
- ready to paint! well, almost. you want to re-do your masking. make the exposed area bigger, you will paint the whole primed area, plus you will need to blend the paint into the old paint with the spray gun. so leave enough room to do that. if you want you can just mask around the entire panel at this point. the entire panel receiving clear must be cleaned (blow off and wax and grease remover), then at least scuffed with 400-600 paper or equivalent scuff pad until all gloss is gone. blow it off and clean it again after sanding. you cannot paint/clear on a glossy surface. it will not adhere well. tack cloth the area to get all dust, then PAINT! WOOHOO! paint the repair until completely covered while blending into the surrounding paint.
- after paint, you must clear within a short period of time (usually 24 hours or so). your entire panel wil need to be cleared, or you can clear/mask up to a line or crease in the body. the reaon for this being the clear will leave a un-repairable hard edge where masked, and it looks ugly. so usually just re-clear the entire panel being repaired. think about this when doing multiple rust repairs. spray clear.
- if using single stage paint, this will be a bit different of couse. but I reccomend Bc/CC over single stage. it is more durable and has a finer finish.
- if you want, after everything is dry enough, you can wetsand the clear with progressively fine sandpaper to get rid of orange peel or runs etc. this will really make that new clear fine. you want it :o there are some tricks for this stuff. ask if you need it. basically, just wetsand with a block, starting maybe with 600 grit, maybe starting with a finer grit depending how comfortable you are. be careful, you don't want to go through the clear. if you start with 600 do that, then you move to a higher grit, higher, higher, higher, etc. ending somewhere around 1500 grit. after wetsanding, buff with a wool bufing pad, then with a foam one. it will be very very shiny and nice. DO NOT wax the new clear for at least a few months.
well, that i think is about my method. i'm sure i have some errors/forgot some stuff typing such a long thing, feel free to correct me. or bash me. or whatevz. also I know everyone has their own way and preferences of doing stuff. so chime in!
to OP, or anyone else, I hope this can help in some way! if you have any quesitons, feel free to ask! :) just want to help.
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