Preparing bare metal for paint

[TS69GT500]

Why is it always recommended to put light sand scratches in bare sheet metal before priming?
I know it promotes adhesion but the factory never did this.
If you've ever stripped original paint using chemical stripper as I have many times you will not find any scratched sheet metal.   

There are plenty of original paint gems around showing no signs of peeling or lifting. Case in point, the Bob Perkins collection.
With the vast improvements in paint/primer chemistry what is the need for scratching virgin sheet metal???

I've only painted panels not a complete vehicle and my paintwork is as solid as the day it was applied and
it has been 20+ years.
On sheet metal I've used a Metalprep type chemical etching as per label instructions followed by PPG DP74.
On my original 69 Shelby fiberglass front valance after chem stripping and without any light sanding I used PPG DP50 and again no hint of an issue after 20+ years.
I was told to clean the fiberglass well and the DP50 will grab like hell.
If you've ever chem stripped original fiberglass panels you'll see they are as smooth as a baby's ass.

Just my thoughts and limited experience.
Thanks,
TonyS     
   

[Bob Gaines]

Are you going to paint the car or have someone else do it?

[TS69GT500]

I would have a pro paint the car and that would mean focusing on the handful of A-list individuals/shops we are all familiar with.

I wouldn't have anyone run a 80 grit DA sander over the sheet metal or god forbid original
fiberglass.
You can kiss your high crown panels goodbye.
Just to emphasize, I did not sand my original factory SMC fiberglass however aftermarket replacement panels with a gloss gelcoat would have to be sanded before priming.
Concerning sheet metal, I've used most of the etching phosphoric acids, Metalprep, Sherwin Williams Dual Etch,
Dupont's ,the PPG product (can't remember their names) since I was a teenager and always had excellent results.
Just follow the directions.

Thanks,
TonyS
 

[Bob Gaines]

I would have a pro paint the car and that would mean focusing on the handful of A-list individuals/shops we are all familiar with.

I wouldn't have anyone run a 80 grit DA sander over the sheet metal or god forbid original
fiberglass.
You can kiss your high crown panels goodbye.
Just to emphasize, I did not sand my original factory SMC fiberglass however aftermarket replacement panels with a gloss gelcoat would have to be sanded before priming.
Concerning sheet metal, I've used most of the etching phosphoric acids, Metalprep, Sherwin Williams Dual Etch,
Dupont's ,the PPG product (can't remember their names) since I was a teenager and always had excellent results.
Just follow the directions.

Thanks,
TonyS
 

Tony ,you kind of evaded answering the question. The reason why I ask the question was that if you were painting the car yourself you can make the decision that makes the most sense to you. If however someone else is doing the car a typical professional shop will want to use their materials and follow mfg. procedures to ensure insurance that the shop and the paint MFG will stand behind the finished job. That means from the bare fiberglass or bare metal up. The surface job is only as good as the surface and or materials under it. If a paint failure and the MFG rep discover that a prescribed procedure has not been followed or non compatible material has been used then you void the warranty. I would be worried about a shop that didn't care about doing the job from bare surface up because when push comes to shove I don't believe they will pay for a failed job if the disaster happens. But that is just me.