Trailer underside treatment

Asphalt-based undercoating has the problem you have experienced: It can lose it's bond and trap moisture rather than protect from it. Better to use rubberized undercoat. Even better, consider applying bedliner plastic.

If you do not expect to be towing it in the winter over salted roads and slush, I'd consider just having it properly painted, over a zinc-based primer.

Rax, can sandblasting remove that asphalt coating?

not well, if at all. that's why sandblast resist is soft. you might have to scrape/burn it off

regards, charlie

I would give it a couple of coats of zinc-based primer as another has suggested. Best place to get it is W. W. Grainger if there is one near you. Their house brand is cheaper than others and it works fine. Buy a case of it; it's cheaper that way, you'll use half a dozen cans on a trailer and it is very handy stuff for welding projects in general. I use it on all steel weldments that will be outdoors and most that are indoors in the shop like racks, carts, etc. Some of them are over 25 years old, still are rust-free and look good. It's no good under "color" as on a car, but it doesn't need a topcoat. My trailer (in metro MN, lots of winter salt) is painted with this stuff, is now several years old with no sign of rust anywhere.

Then you might consider applying some 3M "BodySchutz". This is a very tough rubber-based black undercoating, considerably tougher than the sticky stuff.

It adheres tenaciously, and it shrugs off rocks as on gravel roads. It is possible to shoot color over it, though it is definitely not necessary as on a trailer undercarriage.

This stuff is way too thick to shoot thru a regular spraygun but the applicator is cheap -- or it would be easy to make one. HF used to sell one for about 10 bux, may still. It's basically a crude siphon feed, more like a sandblaster than a paint gun. It might be possible to apply it with a brush, don't know.

can sandblasting remove that asphalt coating?

Try a solvent - paint thinner, kerosene .. sandblasting doesn't work worth a dang. We used to use asphaltum in the shipyards sometimes sold under the trade name "flote kote" or "float coat" or something. It smelled a whole lot like the asphalt driveway sealers you'd buy at H*mo Depo. Anyway, sandblasting didn't work well at all, just smeared it around. I think we had to scrub it off with brushes and solvent.

GWE

Don, so, how would you remove that tar to paint the tar coated metal? Or are you suggesting to paint only newly installed steel frame?

This is very nice stuff. I am going to definitely look into it. I checked out that webpage.

For a permanent fix, go to the nearest Rhino Liner dealer and have the steel surfaces coated. You will never need to worry about rust under the coating no matter where you use the trailer. Gravel or rocky roads will not damage the coating. If the coating somehow becomes damaged like in a trailer wreck or something, it can be repaired like new.

Randy

Yes... In fact, I need to do the same to my pickup truck's bed... That said, I have this problem of what to do to remove that asphalt undercoating. I cannot really use kerosene or solvents for such a high volume job.

Not unless it's real old and dried. You usually have to heat and scrape. Not fun Or, if it's real cold, you can tap it with a hammer and break it off ;) I've used an air chisel on some particularly hard stuff once.

Heat gun?

So, you think that it would stick to the asphalt that is left on the trailer bed, is that correct>?

same to my pickup truck's bed... That

Paint clean steel. If the tar is intact I'd leave it alone rather than mess with trying to remove it. If it is really tar there are solvents will remove it, but it's a mess.

I expect you are dead on that a big part of the problem was the absence of drainage from those rusty members. At the very least, you need to add drainage holes to the rusted members.

Unfortunately, the only reliable way to stop rust after it has started is to remove it entirely. Abrasive methods like sandblasting and electrochemical methods like this:

I don't see how to do either easily on the inside of a hollow structural member.

Your best bet is to cut them off and replace them with new steel, with drainage holes, primed with zinc-rich primer and then coating with the tar-based rustproofing material.

Sandblasted steel should be primed as soon as possible. Often even with a moisture separator (what the heck are they called?) inline, there will be some moisture in the air used to blast so that heating the surface immediately before priming helps.

Ignoramus, A very wise older gentlemen once looked at a trailer that I had just finished building out of 2 x 3 tube. He noticed that some of the frame members were completely sealed shut due to the construction of the trailer and thorough welding. He said that internal condensation would rust the members through far faster than normal unless I drilled a vent hole in each sealed member so air could get inside the tubes to dry them out. I followed his instructions and promptly drilled a 1/4" hole in each frame member that was sealed by welding. Twenty five years later, that same trailer is in service and none of the frame members are even close to rusting through. Incidentally, the frame is painted with conventional enamel and primer.

Bob

Bob, thank you, I will DEFINITELY drill such holes.

igor

I bought the folowing coating from McMaster for painting the frame:

And if you plan ahead and want to go the other way (and you don't mind a bit of overkill) build it the same way they build steel roller coaster tracks...

Make the frame all tubular steel, cap off all the open ends and deliberately drill internal holes leading into both ends of all the structural rails and cross-members. Seal weld all connections.

Place a few pipe flanges and cross-purge the whole frame with dry nitrogen, then plug all the fittings but two - put a small pressure gauge in one and a Schraeder fill valve on the other (with armor rings welded on sliced from frame tubing so someone doesn't wipe them out) and leave 10 to 25 PSI on the inside of the frame.

Won't rust on the inside if there's no moisture inside. And it will give you early warning if problems develop - if it loses pressure, you know to start looking for the crack in the frame or the bad weld.

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If the tubes are *truly* sealed the hole will promote corrosion. The small amount of moisture in the initial charge of air will be consumed in forming a very small amount of rust. (I think the oxygen in the trapped air is also required to form rust, I have no idea whether the oxygen or the water would be the limiting factor, but in either case there's a limited amount of oxygen available.) No exchange of moist air, no more rust.

Ned Simmons

That is seriously good stuff, much better than the rattlecan stuff. If you spray, use a cheap (