Bondo and similar auto body fillers are made of polyester resin and various fillers such as talcum powder and 'microbeads'. It's not very strong structurally and tends not to stick to really slick surfaces. It also doesn't stick to galvanized metal unless it's been roughened up with coarse sandpaper and well cleaned.
A stronger version which sticks better has chopped glass strands and less filler. A common name is Bondo-glass. You can brew up your own by chopping up some glass woven roving and adding some polyester resin to standard Bondo. The cream hardener for Bondo also works for polyester resin. Test a small sample to be sure, there are some variances in the chemistry used! Polyester resin uses liquid Methyl Ethyl Ketone Peroxide while most bondo-alikes use Benzoyl Peroxide (yup, same stuff as used to treat pimples, but don't Bondo your face to cure acne!) as the active catalyst ingredient.
Polyester resin is the polymer in most GRP, Glass Reinforced Polymer. Fiberglass is a trademarked brand name, like Bondo. To get a smooth surface when making a GRP mold and items in the mold, there's a special resin called gelcoat. It has some filler and colorant and is formulated to cure harder than the normal resins. Gelcoat is typically thinned with acetone and sprayed on. I thin some standard resin and spray it over the gelcoat then let it set to a tacky state before starting the glass layup. That helps prevent pinhole bubbles and print-through, which is where the texture of the glass shows through the gelcoat. Gelcoat can also be sprayed or when thickened can be brushed or even applied like spackle to a rough surface then sanded and smoothed and polished to a high gloss. (Back in the day, many of those ultra shiny show cars' bodies were hiding a rough surface under a thick layer of hand shaped gelcoat.)
There are three main types of polyester resin. Finishing resin which contains a wax that floats to the surface to seal it from air so the surface can cure. Air inhibits the curing of these resins. Next is laminating resin which doesn't have the wax. Its surface will take a long time to cure, it can be forced to cure with heat but that can damage the resin. The benefit of this is creating a solid mass of resin that won't delaminate. The third type is called universal or dual purpose. It has wax but less than finishing resin. It will fully cure without applying heat but surface cures slower so that subsequent layers will adhere. Production shops don't use this resin for laminating because the layer to layer bond is weaker than with laminating resin.
Something very important to know about Bondo and polyester resin is they are styrene based and will dissolve plastics containing styrene. That's why they stick extremely well to sheet styrene and will dissolve styrofoam (expanded polystyrene) like Alien blood on a spaceship deck. (I figure that's what they did for that effect, painted a piece of styrofoam then dribbled yellow dyed liquid model glue or gasoline on it. Yup, gasoline also dissolves styrene.)
If keeping the styrene based parts of your case un-dissolved is important, then epoxies are the glue for you. Epoxies are generally stronger that styrene based resins and typically mixed in a 1:1 ratio. They're also more expensive. If you're putting together a kit car, a GRP body made with epoxy will cost more but will be much stronger and resistant to cracks. Same deal when making a GRP portable game console case. JB Weld, Loctite Weld, Araldite and similar adhesives are epoxies with finely ground metal filler. Some rather expensive types have enough metal filler to be conductive.
You can use polyester resin and bondo safely on styrofoam if you first coat it with epoxy. You'll want to do at least two coats of epoxy, allowing time for full cure of both coats. There's nothing quite like watching polyester resin find its way through a thin spot in the epoxy coating and act like a liquid termite on the foam you thought was protected by the epoxy.
Of course y'all know using carbon fiber instead of glass makes the composite even stronger and lighter, no matter which resin you use. You don't need to use glass or carbon fiber. Kevlar fabric will work. (Great way to recycle expired bullet resistant vests.) So will any fabric. Remember the Trabant in the former East Germany? Their bodies were made of resin reinforced cloth. Glass and carbon fiber are just the best combination of stiffness, strength and light weight.
Yes it is nice to be able to put your projects INSIDE something isn't it? You know, to hold everything together so it doesn't flop around? Discuss the techniques here!
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