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Wed Nov 08, 2006 11:22 am

Should I put undercoating back on after a paintjob? Just use Por-15 and not worry about undercoating? or a regular paintjob? powdercoat them?

I'm having them "painted" real soon, just wondering what approach.

My 73 is a daily driver, never a off road car.

Wed Nov 08, 2006 5:31 pm

FreakCitySF wrote: Should I put undercoating back on after a paintjob? Just use Por-15 and not worry about undercoating? or a regular paintjob? powdercoat them?

I'm having them "painted" real soon, just wondering what approach.

My 73 is a daily driver, never a off road car.

I'd vote for a coat of POR-15, plus whatever paint you used on the "topside." Looks best to me.

Wed Nov 08, 2006 7:18 pm

I did the undercoating first....but the entire fender was ready. I think you stand less a chance of somehow getting undercoat on the finished paint or scratching the paint somehow. I used a green bedliner type coating that had rubber bits in it, then I painted the undercoating. I added a flex agent to the paint to lessen the chance of a rock scratching it....If you don't put some undercoating under the fenders, a rock could ding the metal from the backside. I would recommend an undercoating.

I never had much luck with top coating Por-15. The Por-15 must be tacky to top coat it or....you have to scuff it....and thats hard to do, especially in all the hard to get to places.

dog

Fri Nov 10, 2006 8:07 am

We used POR for years until we found KBS Rustseal. It seems to adhere better, and stands up to UV better. The guys in our shop prefer the KBS. Also, our purchasing person likes that it's about 10-15% cheaper! I'll post some photos soon.

www.kbs-coatings.com

Fri Nov 10, 2006 6:02 pm

We used standard PPG epoxy primer under the paint on my dad's. The thinking behind this was that undercoating can hide any new rust that may try to form, and he wanted something that would be easy to clean.

Tue Nov 14, 2006 11:21 pm

If it was mine, I would media blast, primer, and paint the entire thing on all sides with the body color paint. I would then mask off the outside, and undercoat the inside.

And now, I'll justify my answer. 8)

1) Blasting/priming/painting the entire fender (as opposed to undercoating the inside first) will ensure the absence of rust anywhere on the fender, and give the metal a good, uniform protective coating.

2) The biggest argument against undercoating is that it hides rust. Here's the other side:
A) This kind of problem takes a LONG time to happen. The undercoating must dry out and harden, and separate from the fender to trap moisture.
B) If you were even concerned enough to ask how to properly paint a fender, you likely care about your car enough that you would notice the undercoating coming loose, and fix it before it causes any serious probems.

3) Paint is relatively hard, and undercoating is relatively soft. In a fender, undercoating serves multiple purposes- First, it provides a wear surface for things to rub on. Did you run over a stick? Instead of scratching through paint, it rubs off some undercoating. Second, it provides impact protection. Gravel road? Most undercoatings can absorb rock impacts, dissapating the force without cracking or "failing" in any way. Third, in more severe impacts, it adds an extra sacraficial layer. If an impact DOES make it through the undercoating, even if the undercoating cracks away, it still helps dissapate energy. This might prevent a chip in the underside paint (and therefore, prevent future rust), and might be enough to prevent an outward dent if the object was large enough.

My 2 cents.

- David

Tue Nov 14, 2006 11:29 pm

WD-40 wrote: If it was mine, I would media blast, primer, and paint the entire thing on all sides with the body color paint. I would then mask off the outside, and undercoat the inside.

And now, I'll justify my answer. 8)

1) Blasting/priming/painting the entire fender (as opposed to undercoating the inside first) will ensure the absence of rust anywhere on the fender, and give the metal a good, uniform protective coating.

2) The biggest argument against undercoating is that it hides rust. Here's the other side:
A) This kind of problem takes a LONG time to happen. The undercoating must dry out and harden, and separate from the fender to trap moisture.
B) If you were even concerned enough to ask how to properly paint a fender, you likely care about your car enough that you would notice the undercoating coming loose, and fix it before it causes any serious probems.

3) Paint is relatively hard, and undercoating is relatively soft. In a fender, undercoating serves multiple purposes- First, it provides a wear surface for things to rub on. Did you run over a stick? Instead of scratching through paint, it rubs off some undercoating. Second, it provides impact protection. Gravel road? Most undercoatings can absorb rock impacts, dissapating the force without cracking or "failing" in any way. Third, in more severe impacts, it adds an extra sacraficial layer. If an impact DOES make it through the undercoating, even if the undercoating cracks away, it still helps dissapate energy. This might prevent a chip in the underside paint (and therefore, prevent future rust), and might be enough to prevent an outward dent if the object was large enough.

My 2 cents.

- David

Those are actually some great arguments for powdercoating it. 8)

Tue Nov 14, 2006 11:33 pm

Coat the entire bottom side in broken-heart-magenta fingernail polish-

It works great!!! 8)