Preventing rust on underside?

[arabbit]

Hey guys, this is probably the dumbest question I'll ever ask on here, but: recently I found that something put a pretty good scrape on my car's underside on the frame rail. It went through the paint, and I don't want it to rust. What can I do to protect it from rusting? I just took an exterior touch up pen and put some paint on it. It looked terribly mismatched, so I also placed a piece of black electrical over it. Is there anything wrong with doing any of that? Is there something else I could do? I plan on keeping it awhile so I would prefer to protect it as much as I can.

 

[straightliner1]

It's the bottom of a frame rail, don't be overly concerned about matching paint there. If the touch up paint covered the bare metal and is preventing exposure to the elements, you should be safe. If you want to go one step further, you can dab a bit of undercoating or even Rust-oleum paint on the spot.


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[arabbit]

Is it alright to use touch up paint on the underside? In liquid form it's flammable and it is only a few inches from the exhaust. I'm assuming after it dries it won't be an issue.

 

[JTG]

Others have had recommended fluid film on the undercarriage to prevent rust. I haven't used it myself but plan to.

http://www.amazon.com/Fluid-Film-Spray-AEROSOL-CAN/dp/B001OKBFBS

 

[F18SMM]

Prior to applying any form of topcoat you must make sure the area is clean, if any oxidizers are left on the metal it will continue to corrode even under the topcoat.
Not knowing how long the scratch was there before you noticed it, I would remove recently applied paint with fine grit sandpaper until the area is shiny, clean with alcohol or acetone, by using the two rag method. 1 wet with chemical, 1 dry. wipe before chemical dries. Then re-apply primer or topcoat

 

[M91196]

Others have had recommended fluid film on the undercarriage to prevent rust. I haven't used it myself but plan to.

http://www.amazon.com/Fluid-Film-Spray-AEROSOL-CAN/dp/B001OKBFBS

I have used Fluid Film on multiple pick up trucks with no issue except one F250 the paint on the frame rail was attacked by the fluid film.
YMMV.

I use Seymour Rust Converter Primer with much success in similar applications now and paint over it. Not sure I would use any of them on my summer only Mustang.