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Paint flexifier

Once more I’m trying to paint the cowl vent a bear bent in 2000 (so much that the ends touched). I’ve painted it at least 3 times over the years, but the paint keeps on cracking, down to the bare metal, so it rusts. I tried Flex Seal last time: no luck.

I saw a video of a professional promoting a paint additive that made auto paint flexible, but I could find it only in large quantities. I’m going to try liquid paint this time. Is there an additive that would help? Is that Plasti-dip stuff good for this?

The only thing I know of is the flex agent added to automotive paint for use on plastic bumpers. It makes the paint more flexible when the bumper moves a little and the paint doesn’t crack. I’ve never used it and never had a problem but yeah may have to buy a gallon’s worth. You can buy car paint a pint or quart at a time but usually the additives are in different packages. Nothing is cheap anymore though and the last pint of pearl green I bought was $80 with the required additives, and that was a few years ago. Not sure if you can get the flex additive in a spray can with the touch up paint from those touch up paint guys.

I would suggest though that the cracking is from an unsound part so maybe add some reinforcement to it or something so it doesn’t flex. So address the prime cause. The thing with plasti dip or something is that maybe the coating will stay intact but it won’t be sticking to the metal anymore so what good does it do?

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Why non use plasti-dip. If you used flex seal. it was probably black…Plasti-dip comes in a spray can in black. If you don’t like it, it peels off.

If the cowl vent is made of metal,use Tremclad rust paint…available anywhere. Por 15 rust paint is much more expensive and goes on thicker.If you don’t want to do it again,use a self-etching primer before you apply the rust paint.If the cowl vent is made of plastic,use an adhesion promoter before you paint. If you want to skip this step,use the black trim paint made by dupli-color.

I had hoped that this was what Flex Seal was.

Who are the ‘touch up paint guys’? Pep Boys, Auto Zone, Walmart have only special kits for touch up paint, $15 for ¼ ounce in a special dispenser.

Didn’t know that; never used it. Thanks.

They make clear too. I put a coat of red over it.

What if I do like it?

Good idea. I thought I had made it as stable as I could. Considering that this is an '87 that a few people have run into and I’ve fixed myself, it’s non-beautiful. I was hoping I wouldn’t have to replace it.

I’ve used Dupli-color’s Rust Fix. I sand down to shiny metal.

I apply a couple of doses of Krud Kutter The Must For Rust - is that the same?

I don’t think rust is the problem: when the paint fails, the metal underneath is shiny. It only rusts because I put off dealing with it.

The paint you are applying depends on a mechanical bond. Smooth, shiny metal is not the ideal surface. It wants some texture to the surface or “tooth” as it is sometimes called to aid in forming a bond. If you’ve sanded it down to smoothness, that is part of the problem. An etching primer will help…Krud Kutter may not be effective alone without a primer and may not even be compatible with your paint, spoiling the bond…


I believe that if you’re not using primer, that is your problem. Sand, do the krud kutter if you want, apply a couple of coats of primer, sand lightly with fine grit sandpaper, clean with tack cloth, and paint. Or use the self etching primer and skip sanding the primer.


One other thought that I have seen spoil paint jobs is when people start sanding without stripping first. You can actually drive incompatible substances into the pores of the metal surface, spoiling the bonding. An example would be sanding a body panel before stripping all of the waxes and whatnot off the surface first…

1 Like These seem to be the big guys and pretty accommodating. I need to order some myself before winter sets in.

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I don’t make it that smooth. What I mean is I have removed all paint and rust. It’s still pretty rough.

It’s a Rustoleum product; I use it with Rustoleum primer and paint.

It would be if I weren’t. Of course I’m using primer.

The stuff @Tester referenced gets terrible reviews and says it’s for acrylic paint. I’ve never seen acrylic paint for cars.

If the issue is rust and cracking, but not peeling, I’m not sure the flex additive is what you need. Flex additive us normally only used for plastic parts.

What I recommend is sanding it until it is free of rust, priming it, and then painting it with enamel, putting on a few coats, and then a coating or two of clear coat.

If you’ve primed it in the past, there is a primer called “rust converter” that I highly recommend. I often find it at Walmart in the automotive section.

The car doesn’t have a clear coat, but I have always done what you say in the past. Other places on the truck stay painted. Only this wonky spot keeps on cracking.

What I always use.

Urethane paints are naturally flexible. There is no need to add a plasticizer. If you use urethane, prime first. When priming, use a very thin coat. Priming as if it’s a top coat will lead to cracking.

All the urethane varnishes I have applied to wood have been brittle. None of the paints at the auto store are urethane.

I wonder if paint formulated for boats would work. All recreational craft flex.

All the urethane paints we use at work are flexible and none of them need a plasticizer or flexiblizer to make them flexible. The only wiffle dust the manufacturer puts in are the colorant and flame retardant.

Try doing a conversion coating. This is the most common first layer of protection applied at the factory. You need some phosphoric acid. Dilute the acid 50/50 with water by adding the acid to the water, do not add water to the acid as it will get very hot and could explode (steam explosion) and get the acid on your skin or in your eyes.

Wear rubber gloves and use a scotch brite pad to apply the aid to the surface, rubbing to a break free (no beading) surface, let sit a minute and rinse off. Let it air dry. It should have a dull gray finish that looks like tarnished aluminum. Spray a sealer/primer on it. Zinc Phosphate primer would be the best if you can find it. NAPA used to sell it, I don’t know if they still do.

Then paint.

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What kind of work do you do? Can I buy these paints in stores? A search for urethane paints on auto store websites turns up nothing.

Could I use Coke™? Where do I get a little phosphoric acid?

I’ve repaired 2 spots with Bondo after someone ran into me. They should be even less stable than this spot, but the Bondo has stuck.

Actually yes but I wasn’t going to mention it. Don’t dilute it and you have to wait quite a bit longer.