My car is currently being painted. The paint shop owner asked what I wanted to do with the backside of the fender--something I had never thought about. The image above is from another car--the back of my fender is mostly grey with some old undercoating.

The shop suggested these three options:

paint it orange (like the outside)

POR over the old undercoating (there is no rust)

Leave it alone

Any thoughts?... I can't imagine this ever being seen unless you look under the car.

Edited April 25, 201212 yr by Rich1

If the fender has undercoating on it's backside, I would NOT apply POR or paint on it.

If the undercoating is in good shape, just re-coat it with the same undercoating to freshen it.

But if you plan on driving your car at all, through any kind of moisture or gravel, you'll want to coat it with something.

If you do want to use POR, you want to apply it directly to the properly degreased and etched metal and not on top of another coating that will allow it to peel off, and it WILL peel off.

Paint over undercoating is most commonly seen as poor masking.

FWIW

E

Edited April 25, 201212 yr by EScanlon

Remove the old undercoating - you never know what's under it to begin with. It can be removed with Mineral Spirits or even Kerosene in a well ventilated area. Soak it down - let it sit a few minutes, then use a Putty Knife and some rags. It is most likely thin to begin with.

Once the old undercoating is off - Take OSPHO {a mild solution of phosphoric acid} and a Scotch-Brite pad and scrub it down - usually that will result in removing everything down to clean etched metal. Let the OSPHO work for a few hours, then rinse any excess off - -Then paint the inside of the fender with POR-15.

While the fenders are off - do the same thing to the rocker panel where is goes under the fender. Scrub it down with OSHPHO, let it sit for a couple hours - then rinse off any excess. Paint with POR-15.

Keep a camera with you and photo document everything...

FWIW,

Carl B.

Carl, I disagree with you in one respect... do not substitute the POR chemicals for others "just as good".

Time and time again, people complain that the POR process did NOT work for them and after discussing it it turns out they substituted chemicals or processes deemed to be "equal" to the POR's. Those that did NOT, haven't complained and in turn recommend the product.

While OSPHO may be a great product, I can neither vouch nor denigrate it as I haven't used it.

I HAVE used the POR process extensively and at 10+ years have still to find problems. Then again, I used the Marine Clean AND the Metal Ready products and followed the instructions faithfully because I didn't want to waste any of the product. My Z done in 2001/2002 and the Roadster done in 2007 were all done that way and I've not found any problems due to the POR.

2¢

E

Hi E:

I wasn't really recommending the substitution of anything. As you can see a bit farther down in this reply - POR-15 says that Metal-Ready isn't necessary in this case.

I was only outlining what I've done for the past 30 years. In terms of cleaning up undercoatings and treating surface rust on auto sheet-metal and preparing it for farther processing. In this case painting it with POR-15 I've only used it for the past 15 years or so.

I do strongly agree that with any chemical product - one is always wise to read and follow the manufactures directions. So I support your advice.

For that matter my Supplier of Automotive Paints and Supplies - called and talked to the people at POR-15 before the Supplier started carrying the product, to find out exactly what could or could not be used with it. That was at least 15 years ago... when people actually answered their phones.

The owner of the Supply business happens to be one of the best Auto Body & Paint men I know - he has been restoring cars for 40 or more years. So he has lots of hands on experience with automotive chemicals and modern paints etc.

Reading POR-15's Directions/Questions/Answers from POR-15 Site:

- - - - -Quote - - - - -

POR-15 Metal Ready {I guess now called Prep & Ready}. Questions and Answers

Is It Always Necessary To Apply Metal-Ready first before using POR-15 Rust Preventative Paint?

NO - Metal Ready must be used first before applying POR-15 when:

1. Painting on galvanized metal

2. Painting on aluminum

3. Painting on stainless steel

4. Painting on smooth, polished, or new metal surfaces.

- - - - - End Quote - - - - -

I'm sure that POR-15's Marine-Clean and Prep & Ready would do just as good a job. Although I don't know what the cost would be. OSPHO is relatively inexpensive. I believe that some Home Depot Stores carry it now as well. Auto Paint & Supply Stores have carried it for decades... OSHPO is commonly used for the situations 1-4 above as well.

FWIW,

Carl B.