First you need to make sure that the surface you are applying the bondo to is free of dirt and grease, especially silicone. Also use 40 grit sand paper to prep the surface and sand about 2"-3" beyond your repair.
I usually mix up small amounts because the stuff dries fast. I squeeze out the hardener to about the size of a dime and mix it to a golf ball size/amount of filler. Keep mixing it until the color changes and is uniform.
Spread the filler on the repair with a plastic spreader and extend the filler out about 2"-3" beyond the repair for better feathering. It's always better to build up your thicknesses in 2 or 3 applications as opposed to just slapping on one glob. You'll reduce the chances of cracking and it's easier to work with.
After about 10-15 minutes you'll be able to sand it down. This is where everyone has there own preference as to how they sand and the equipment they use. If you have'nt done much sanding, feathering or contouring I'd start off by hand before using an air or electric sander.
I start off by knocking down the filler with 80 grit paper on an air file. Then I move up to 150 grit to further knock it down but at a slower rate. I then move up to 240 grit on an orbital sander to start smoothing out the surface and feathering the edges. At this point I apply glazing putty to fill all the pin holes and continue sanding with 320 grit on up to 500 grit for a smooth finish.
Best thing for you to do is practice on old sheet metal. Go to the junk yard and get you a small hood or trunk lid and practice sanding the metal, prepping it and filling the dents. Mix more hardener in with the filler so you can see the affects it'll have on you repair. Do the same with mixing less hardner. If you absolutely don't want to use a little "elbow grease" and sand by hand, using your test panels is the opportunity to practice with air files, jitter bug sanders, high speed sanders or orbital sanders.
One other thing, when sanding by hand DO NOT use just your hand and the sandpaper. If you are sanding a flat surface wrap the sand paper around a block of wood. If you are sanding a curved or compound surface use a sanding pad. Sanding pads are semi-soft blocks that allow the sand paper to conform to the shape of the body panel. Using a sanding block will give you the best results when sanding by hand.
Not using a sanding block and only holding the sand paper with your fingers will result in a non-uniformed surface. One finger will inevitably push harder than the others creating a groove/rut in the bondo surface.
wow, thanks so much atl. im planning to bondo my rear valances and fix a small crack on my kit. where do you buy bondo? and what kind of tools are needed (sand paper grade)? my goal is to just mold my kit and fix the cracks. Then i'll take it to a shop and have it painted thanks so much guys
Since your molding in your rear valances I wouldn't just use Bondo or a similar filler. You may want to invest in some fiberglass mat and some good flexible and sandable epoxy resin. If I'm correct those rear valances lay across part of the 1/4 panel and you can see a definite edge. Of course that edge is what you'll be sanding to feather it so that it will be flush with the 1/4 panel. That is the area that you'd want to lay some fiberglass. It'll provide additional strength and sort of replace some of the fiberglass you removed from sanding. Once your epoxy is dry then you can sand with the objective of contouring and then go back with filler (preferably a fiber reinforced filler) for your cosmetic touch-up's.
You may also want to think about using the fiberglass mat to repair your crack in the kit. Cracks in fiberglass can migrate quickly and can cause your filler to crack. Best way to tackle that is to get a very small drill bit (1/16" or 5/64") and find the absolute start and stop of the crack and drill at both ends. This will prevent the crack from migrating any further. Then step sand around the crack and 2 inches beyond ie. if the crack is 3" long, then sand 2" from each end and 2" wide. You'll end up forming a 7"x2" rectangle box. With this little step sand trick all you have to do is cut out a 7 1/4"x2 1/4" piece of fiberglass mat and lay it in there flush. Very little sanding and again you'll end up using less filler, only for cosmetics and repairing very minor inperfections.
Tools wise, probably go with a good high speed grinder/sander, an orbital sander, some sanding blocks of different lengths and a flexible sanding board for contour sanding. As far as sand paper is concerned I personally like wet/dry paper because, well, you can use wet or dry
Pick up some 80 150 240 320 500 and maybe even 1200 for some wet sanding, depending on the type of body filler you buy. If you use Bondo DON'T wet sand that. It has the ability to retain water.
Good spot for fiberglass, carbon fiber, kevlar and even that ceramic shit they use on the Space Shuttle. You'll also find good quality epoxy resins. To me, there's nothing better, and more permanent, than using composite materials approved and rated for aircraft. It may be overkill, but you know it'll last longer than your car or your kit.
Hope I didn't bore you with such a long post and please post some before and aft. pics so we can check out the final outcome.