Dabney Gets Bubbled
Dabney spent the better part of a week sanding and prepping the body of his 1965 Corvair Corsa. He had decided to paint the car with his new HVLP spray gun and wanted to test out his skills.
During that time he consulted with the Doctor and both of them were satisfied that the body was ready for color. All primers and surfacers had cured long enough and the weather was cooperating very well for the application of paint.
On the morning of the paint job Dabney did all the proper tasks to ready the car for its color coat, including the wipe-down with solvent, drying of the surface and the
wipe-down with tack rags. The paint was mixed with hardener and reducer and the paint gun's spray pattern adjusted to perfection. The Doctor watched as Dabney sprayed the first coat.
It went very well and Dabney was pleased with the outcome of the first coat. The Doctor helped him mix up the next batch of paint, after which Dabney sprayed the body for a second coat. Both were quite pleased with the results and left the shop for lunch, removing their breathing masks as they left.
"It's a good thing we wet down the floor before painting," said the Doctor, "it looks like there's a minimum of dust on the finish. This should be easy to sand out and buff."
"That's right, Doc," said Dabney, "The new gun laid the paint on smoothly and I didn't see any runs in the finish. We'll check it out thoroughly tomorrow, once it's cured enough to venture into the shop without causing problems."
They went on their separate ways later in the day and agreed to meet up after coffee in the morning. The next day they met for coffee and then went into the shop to inspect the
"What's that?" exclaimed Dabney, as they walked into the shop. "It looks like big dust particles are all over the surfaces of the body.""That's not dust, dear Boy," said the Doctor as he stared closely at the Corvair's finish, " that's millions of bubbles coming up through the finish."
The two couldn't believe their eyes. The entire car's finish was a sandpaper-like eruption of tiny bubbles. It was ruined. "What could have caused this?" asked a disheartened
Dabney. "I don't know, but we'll have to figure this out before re-shooting the body," said the Doctor.
He grabbed a piece of 400-grit sandpaper and gently sanded an area on a fender. He worked with the sandpaper until the area was smooth and then inspected the surface carefully.
"It seems the bubbles came from the first coat and up through the final one," said the Doctor. "The sanding gets us through that layer and underneath it's fine, so
the good news — if there is any — is that the body won't have to be taken back down to bare metal."
"Great, Doc," said Dabney, "but how do we know if the paint is any good?" "We don't yet, old boy, but we will by the time we figure out the problem," said the Doctor. "We have to do some research."
The Doctor went off to his office and left Dabney to start sanding. He turned on his computer and started searching for forums or technical bulletins about paint bubbling problems. Eventually he found the answer by consulting a chemist friend of his at DuPont.
Dabney was sitting in the garage office when the Doctor strode in. "Well, Doc," said Dabney, "do you know anything?" "Yes I do," said the Doctor, "and it's our fault." "Our fault?" asked Dabney, "what did we do to cause this?"
The Doctor started talking. "Well, it seems the problem occurs when there's too little flash-time between coats or the solvent temperature range is incorrect or the final coat
is too thick."
"We were pretty close in those areas but were still within the allowable limits," said Dabney, "so what did we do wrong?"
"There's one other thing that causes the limits to change," said the Doctor, "and that's done by having too much air flow around the job when painting. Too much air flow causes the first coat to skin-up and the solvents aren't released. We were running the exhaust fan at high speed during the painting because the area was getting a little warm, remember?"
"Yeah, we turned the fan on high just before we started the second coat," said Dabney.
"We did, and that's what most likely caused the failure of the second coat," said the Doctor. "We were painting the car in about 80 degrees temperature, which was the outside range of the reducer, and then we increased the air flow. It was the combination of these two that caused the bubbles, but at least we know now. This isn't anything that a new batch of paint won't fix, so let's get to it."
The two of them finished the sanding and then re-coated the car, after carefully making sure the conditions were correct.