undercoating/bedliner

I have found a little bit of spot rust underneath my unibody and on spot about 6x6 inches in the cargo area that has completely rusted through, underneath the carpet/mat. I've been on youtube and google myself to tears so I'm turning to you guys. Is it possible to DIY re-apply and undercoat to a vehicle without a complete strip or sand down to the bare metal? Bedliner type products seem like the best option for all around resistance and sound deadening??? WHAT IS factory undercoating??? I live in the swamp so I want to be "pro-active on a budget" against water intrusion and rust. ANY links to how to videos would be greatly appreciated. However, I'm looking for real world experience!!! :compwork:

Undercoatings seem to take re apply well. I just wash it the best i can and let it dry and then use a wire brush to get the rest of the dirt off and scuff up the surface and use the air gun to blow it all off.
If you want it to stop then the rust/rot needs to be cut out and replaced. Then prime and paint the hell out of it. I love and hate undercoatings for the fact that its a great barrier from impacted damage that would chip paint and then rust. But if water makes its way between the undercoat and steel then its going to rot out and you'll never know to the floor is soft. The coating hides the fact till its to late. I like to prime and then paint with 3 or 4 coats of both then under coat after a good week waiting time. That way the paint is %100 dry. Undercoating protects the paint from rocks and the paint will protect the steel if the first line of defense fails.

Concur with Bugout. In addition, there are some other ideas that other's will probably add to this discussion. !) Obtain some OSPHO, a blue/greenish liquid of phosphoric acid. Get a windex type spray bottle, and wash all your windows. When bottle is empty introduce the OSPHO into it, and be sure to re-label it, and store away from accidental procurement. Wear goggles, and a face mask, bandanna will do, and perhaps a disposable paint suit, or clothes you do not care about. Spray all rust, (after you have mechanically removed all that you can, i.e., wire brush, etc.), with the OSPHO, and let dry a day. Wire brush it again, and re-apply the OSPHO spray. Wait another day, and then begin to prepare the surface with the painting layers as have already been described. Or, cut out various pieces, and replace with some pre painted galvanized sheet steel, but do not torch/weld on it as the zinc can be harmful. Alternately, use raw steel, and paint that up instead. You can, if the section allows for it to drill holes, and bolt it all up with washers, nuts, and run some silicone around the edges of the patch thusly acting as a gasket. I've used a bead of Liquid Nails, or some plywood construction glue as an inner bead surrounded by the silicone bead. Think rings of a target, with the bullseye as the rusted out hole. Round, or square target, lol, alternating the rings with the stuffs.

The OSPHO simply turns red iron rust oxide into a gray/black inert iron oxide. The primer paint, then paint, (there are some rattle can paint companies that also have selections which contain the extra toughness of beads of glass, etc.). Once painted, even simple under body sprays will work ok, using several coats over the afflicted area. Two years ago, the XJ I bought, one could see the road next to the clutch pedal, and door. Fixed as described above, and absolutely no problem since, going through rather extreme temperature extremes, as well as preventing intrusion of water, sand, salt, snow, air, etc. So, the patch I did has so far lasted two winters, three summers, with narry a problem.:repair:

X2 Be sure it clean like mach2nh said if water get between the body and undercoat with no place to go it going to rot and you will never see it. And as far as tear down you should be able to do it with out a full tear down. But I know there are some think I would pull apart to make sure I get it done right. Make sure you get it clean and get rid of any rust before start putting it on.

A full tear down is not an option for me right now. I don't have the facilities of the space to do that. This is my plan. Tear the carpet in the rear out ASAP. Since the rusted through spot is so small and pretty easy to get to from underneath as well, I'll cut out the really rusted sections around the inside of the circle. Then sand/grind that area down to the metal. Several coats of primer and several coats of paint on both sides. Then do the same to the to a piece of galvanized sheet steel. Lay in the steel with a butt load of RTV. Then spray the underside of the new spot with some bed liner or something like that.

That will get me through the rainy season.

Come September, Ill Degrease and pressure wash the underside till I'm blue in the face. Put it op on jack stands, take the wheels off then grind and sand every little square inch I can get too with an angle grinder, drill with and extension or even a dremmel. Pressure wash again.

From then on I'll go with Alaskan89XJ advice or use some POR15...Is there a difference?? Then apply whatever undercoating I've decided to go with by then after a week or so, I'll throw some kind of roll on/rattle can combo bed liner.......

Feelings, questions, comments, concerns and PLEASE suggestions????

I would not pressure wash after grinding. I would wipe down with acetone. I mean that stared the problem. But thats just me.

Watch out for Undercoating/bedliner because if you clog a drain, it will collect water and rust. Do not cover rust with it, it just makes it worse in the long run.

For your floors:
.Cut out rust.
.Clean with Xylol.
.Apply coating of rust converter to exposed edges an allow to dry.
.Get a tube of seam sealer and seal the edges top side and from underneath.
Allow to set.

.Prime and paint underside side.
If you are so inclined, use a roof sealer on the plate overlapping about 1" on all sides. This stuff comes in a 5 gal can and gets applied with a spatula. It is oil based and is a PITA to get off your hands except with gas.

.Top side:
If you really want herculiner, now is the best time to apply. while you have the carpet up. Don't use spray can stuff. Based on past experience, I don't trust the stuff not to be porus.

If you don't want to spend the time and effort to herculine, prime and paint the inside also. When it is time to herc in the future, it will be ready to go.

What is rust converter??? Is it just called rust converter at the store??? Ive never done any body work. Ive always been afraid to for the same reason Im not a carpenter....things I make rarely look pretty when Im done....

X2 on using seam sealer

If I had to rush it, I would cut out the rust, rust converter and oil paint the edges and patch, then add patch with seam sealer and rivets or self tapping screws