I’ve used Bondo since my Old Man had me sanding in the garage age 12. Best price around here was at Wallymart before all the World Drama. My grandson thinks I’m crazy, says Bondo Brand is “ TO HARD TO SAND”! Is he starting to show his feminine side? I can’t tell these days. Maybe Drywall mud would be a softer sand.
Bondo is bondo is bondo, all polyester filler made from a talc known as cabosil (sp?) and polyester resin. The more resin the more prone to shrink on heavy fills. Less resin has bonding issues. I've found no value difference in Rage Gold vs USC Chromate Light. My wallet saw quite a difference as the Rage at the time was triple the price. Sanding? Same on both, and it's your timing that makes the difference. They're all hard as a honeymoon dick the next day save a few cheap junk brands. Worst of the worst for next day is Duraglass fiber reinforced. All fillers are fairly porous too and will absorb humidity given the chance. This test should be a reveal if you look hard enough or have too much time with these products in your life (over 40yrs for me). Clean and proper mechanical bond is essential. Filler doesn't like acid etch only, in some cases it will peel off like mold release. Even just bare steel treated with metal prep only will peel off, a mechanical bond is as essential as clean is to welding or solder. Last, filler with extra resin will soften almost all paint related base layers. The solvents in the filler need time to purge out much like coats of paint, so again conservative use is best. And yes, contrary to popular belief fill CAN be blended into a painted area with no detrimental effects. Well, unless it's been piled on. Good stuff here. Big tells...
A salt fog test demonstrates that moisture actually penetrated the epoxy primer. a few years ago on another formum someone asked that they had put epoxy down a couple weeks prior asked what to do to get better adhesion in areas that cannot be sanded. I recommended a solvent wipe. I was told I was nuts. So I took a panel that was 2 weeks old with light grey epoxy primer. Yellow rag with urethane reducer and wiped the panel there was gray on the rag and I posted a picture on another forum. It caused a major shitstorm. I was run out on a rail. Epoxy is recommended under the water line over urethanes. Things that we deal with are moisture , oxygen , solvent resistantancy and corrosion resistance. Moisture cured urethane primer will do 10 days immersion in acetone , when the acetone is removed it will reharden. A full polyester urethane will do 90 days immersion without softening. Both epoxies and moisture cured urethanes are waterproof to a degree The addition of aluminum, micaceous iron oxide, mica is used to cut off oxygen. The new epoxies from Europe contain glass spheres , or mica to cut off oxygen to improve corrosion resistance.
With Bonbo I always leave it at least overnight to set and gas off. Also had better luck sanding when completely set . Just seemed to go easier. maybe just me!
The Jalopy Journal
