I'm not a great painter, but have this one experience I always try to remember and pass on, regarding wet sanding: if Bondo (or similar) filler is used, don't ever wet sand it. It never dries. About 1-2 years later on a hot day, it will "steam" and neatly crack the paint in the shape of the Bondo patch, then lets the paint fall off. Also neatly, outlining the Bondo.
Once I found this out, it completely explained what happened to my 1972 El Camino (new, from Chevy) in the winter of 1972. I bought it in September of 1971, custom-ordered from the factory as a hotrod (big block, 4-speed, posi, wheels, the works), took 6 weeks to get it (a long time in those days!). I noticed an ever-so-slight ripple in the black stripes that graced the hood, but didn't think much about it, was all excited to try out the car/truck, took delivery, drove away smiling.
Short-lived smile.
The short story (believe it or not, this IS the short story):
A week later, back to the Chevy dealer: the front end was wacko, car wandered all over the road from day One. Turned out, the subframe was welded together crooked. I watched as an angry, frustrated mechanic beat the lower subframe with a 4-pound sledge hammer so he could make the front end align somewhat close. Once he missed, and bent the 5-spoke left front wheel on the inside rim. Had to make them get me another: it came unpainted - they painted it, but it did not match the other 3, and they refused to paint those to match it.
Month two: the clutch starts slipping. Rear main oil seal popped out, drowning the clutch in oil. Engine not even broken in yet (2000 miles), gets pulled out for the seal. Sits 3 weeks in the shop, waiting for the seal. Snow starts, still riding my CB750K1.
End of November, 1971: haul my CB750K1 in the truck to Peoria for my friend Jim to find out why it is handling so badly (whole 'nuther story of mis-made frames...). Enroute, the fan belts all fly off the engine, just 40 miles since I got the truck back from the shop, 11 PM in a small farm town. A wonderful garage owner comes out to meet me in his garage at midnight, puts on new belts for me. I get to Peoria (40 miles more), then back to Macomb (80 miles back), all belts fly off again as I enter town. Install new belts, a little looser, drive to Missouri next day: all belts fly off about 300 miles later. I have spares. At the old farm where I was raised, discover the power steering pump was installed into the wrong holes on the engine bracket - by the Chevy dealer who pulled the engine - tilted it about 7 degrees, eats belts. Fix that.
Close the hood: notice the paint is cracked on the black stripes. In 5 places. Before I get home the next day, 3 of those cracked spots are bare, to the Bondo that was used to smooth the hood so it looked pretty. Now it is December, 1971. Go back to school in January, -20 degrees all month. Come March: 8 more places on the back of the car are shedding their paint, all showing Bondo underneath. Take it to the Chevy dealer: sorry, paint only warranteed for 90 days. Pissed, go back to school and work.
The next summer (1972), I take it to a painter to find out why this is happening to my new car (8000 miles on it). He explained very patiently, knowingly, and shaking his head: GM uses Bondo-like body filler, wet-sands it to shape to smooth out their crummy stampings, dries it under heat lamps (sometimes) for 24 hours, then paints it. The Bondo holds the moisture until an opportune moment, then bubbles the paint. He makes most of his living fixing GM cars, repaint work with no Bondo. Uses epoxy filler instead, dry sanding only, but wants as much as I paid for the car to fix mine. I don't have the cash.
I sell the car, still with bad paint, engine has been out twice for rear main seals, shifter broke off inside the cab (I replaced it myself, won't let the dealer touch it, now), posi clutches gone because dealer used wrong oil at the oil change (scheduled warranty maintenance), paint falling off the (mismatched) wheel and steering pulls hard to the left (on 2 lane roads!), no more adjustment left in front end. It was 13 months almost to the day I got it, sold it for 20% of what I paid new.
And, it never got more than 10 MPG, on the hiway. That was my last GM.
And, this IS the short story of it...
