rusty/metal surface prep

YOu guys were so helpful b4 with painting suggestions/recommendations, how could i now come one more time for your help?

After my tractor is painted, i wanted to store it in a more "dust free/debris free" shed. Therefore, i am first putting felt paper down over a cedar-boards for the vapor barrier, then applying second-hand corrugated metal.

I am reversing the older more rusty side and putting the better half facing-out. However, there are still a number of rust spots and some places of heavy oxidation in general.

I was thinking of buying a couple of gallons of Phosphoric acid and applying it with the pump-up/garden sprayer. I was first going to take a shop broom with heavy bristles and brush the roofing sheets [while on the bldg] to remove any/as much of the flakes/etc before hitting with the H3PO4 acid. Here are the questions:

1). What is the best way to prep the bldg [sheets are already screwed on]?

2). Should i use a primer, or can i just hit with the "barn-red" paint after cleaning the surfaces?

3). Can you recommend a type of spray-gun for this type of application? [i have no spray-guns, but a real nice 5-gal compressor...if i can get a spray-gun that has interchangeable "heads" for different appl's, i would pay more for that if you think that is the way to go, like future tractor painting]

4). If H3PO4 is not the way to go, is there something else that is economical or better? What type of primer do you recommend and what type of barn-red paint?

Lastly, the humidity or dew point in the evening/night is sometimes overwhelming, so if i prep'ed it in the morning [no heavy winds], hopefully you can recommend a process that would allow me to cover the prep'ed c-metal sheets before night so the dew will not precipitate a surface rust.

Thanks a million guys - who knows, after i get good painting buildings, i might start painting my own tractors.

Appreciate your help,
tstex

I assume your corrugated roofing is galvanized, with some rust spots. I haven't done it myself, but here's how I would approach it if it were mine. Wash it all first - power wash preferable. Wire brush all rusty areas. I assume the phosphoric acid you are considering is a dilute solution intended for a paint prep purpose. Use it not only on the rust spots but all over. That will "etch" the galvanized for better adhesion. Follow the label instructions. Some phosphoric acid pretreatments are kept wet (so do relatively small areas at a time) until they can either be wiped dry or flushed off with water. In either case, after applying with your garden sprayer, scrub it in well with a Scotch-Brite pad. Wear rubber gloves for this of course. I haven't applied such stuff to galvanized in years but I recall noting a reaction when using it on galvanized rocker panels a long time ago. I can't remember if it smoked or not but I do remember I was suprised at a reaction. The product I used was DuPont's 5717S, which is a phosphoric acid-based product intended for use as a paint pretreatment, not only for steel but also for galvanized. You might want to consider that also. I don't know how it differs from the many other similar products.

For a paint, I'd choose Sherwin Williams exterior Duration. It's an acrylic latex that I have had very good luck with on both brick masonry and wood:

It's a premium house paint that is not cheap - probably about $45 per gallon now. It is available in flat, semi-gloss and gloss in many mixed colors. It doesn't require a primer on bare wood (the first coat serves as the primer) but it does require a special primer on brick. The tech sheet indicates galvanized as an acceptable substrate. Anyway, follow the instructions. That paint is thicker than your typical house paint and should not be thinned. It is best applied using airless spray, but also can be applied by brush and roller. I have used all three application methods. It dries very fast. In your case, w/o airless spray, I recommend a small diameter roller perpendicular to the corrugations. Do it on a cool day without sun and with no rain expected. If not, it will be impossible to keep a wet edge. Since it's a latex, I'd apply it in the morning or early afternoon because a heavy dew shortly after application could mess things up. I'd try for the acid treatment first thing in the morning after the dew has dried up and then apply the paint right after lunch the same day as the best schedule. But make sure everything is completely dry before applying the paint. I'd use two coats of paint, the second coat the following day since there is a minimum recoat time of 4 hours.

I'm not positive that the above direct link to the Duration tech sheet will work for you. If not, go to the Sherwin Williams website and poke around until you find it. You'll have to poke since it's not readily apparent how to get to it. I think it's called a "data sheet". They are in the same area as the MSDSs are.

Rod

Thanks again for your help! Where do you live? That is a great shot of a deep blue-sky.

First off, i completely was off-base on the compressor; it is a 15 gallon and can get up to 160psi if needed...i usually have it set at 50-60 psi for most of my applications. So with that in mind, i believe we should be set with a spray-gun.

Do you have some specific recommendations for a universal type spray-gun? I live in Houston so i can get access probably everything?

The sheets [10' x 3'] were probably galvanized, but they do not look like it now [the outer side that was facing out are all pretty rusted, but they still has good integrity - no where to rusting through]. When i do something, i like to do it right and will always spend the extra time. However, scotch-briting the whole thing would take an enormously long time. The bldg dimensions are 75' x 7' = apprx 525 sq ft. That's a lot of scrubbing.

Would it be okay to first power wash with TSP, dry, then take a lg wire brush and hit any heavy/flaky places, then hit with shop brush and/or heavy air from compressor, then hit with phosphoric acid [kind from H-Depot, gal size in green labeled container].

Based on this scenario, would that be okay? How much paint would someone need for 550 sq ft?

Thanks again Rod, i appreciate your recommendations.

regards,
tstex

I'd still forget the compressor for this particular job. Even if it could support a full-sized conventional automotive type spray gun (which is not clear with the information you've provided) it would be like painting a barn instead of a car. The atomization would really be too fine for the intended purpose and the compressor would be taxed extremely hard. Besides, the Duration paint is too thick for any conventional gun and it is not to be thinned. An air-atomized spray gun is not recommended for that paint by the manufacturer, Sherwin Williams. Both that paint and your application should be handled with an airless spray outfit - or a manual roller. The airless spray would make relatively fast work of it. That's what is commonly used for painting houses anyway. You might want to investigate renting one for a day. I painted a 400sf porch ceiling a couple of years ago with one. The actual painting took only about 10-15 minutes per coat. It took quite a while to clean the equipment however. Flushing the hose is not a problem but disassembly and cleaning the individual pieces of the gun is a pain. Not something I'd use for a small area, but for your job, it would be ideal, even if you use another paint. The chimney was done with a roller, due to both relatively small surface area and the difficulty of overspray protection for the surroundings. Before I bought my airless sprayer, I investigated renting one. I was thinking about an oil based paint at the time and the rental agency would not rent me one unless I used latex - too many people return the equipment w/o proper cleaning. Using oil based paint requires solvent cleaning whereas with latex, equipment cleaning can be done with water.

Don't know what to tell you on the phosphoric acid. Yes, that's a lot of elbow grease. But paint adhesion to galvanized can also a problem. Whether a phosphoric acid treatment w/o the usual scrubbing will provide an acceptable adhesion for the topcoat, I just don't know. I'd be nervous not to scrub it. Check the instructions on the label of the specific treatment you plan to use.

The tech sheet for Duration indicates a coverage of 250-300 SF/gal/coat. I think that's with an optimum application, a uniform film thickness and no unavoidable waste. Personally, I'd figure on something less than that - say something like 200 SF/gal/coat. I'd much rather have paint left over than run short.

A lot also depends on the proportion of rusty areas to galvanized and the severity of rusting present. I don't have a lot of faith in any of the phosphoric acid treatments dealing properly with "significant" rust, even after wire brushing. I am going to be painting a presently rusty 3pt hitch snowblower later this month. Here's the second stage impeller assembly:

Which is typical of the entire blower:

I will not be sandblasting it - only quick wire brushing the worst of it. And I will not be relying on a phosphoric acid treatment for dealing with the remaining rust. I will be using POR15 for the first time, brush applied, as a base for an automotive acrylic urethane topcoat. I'll see how it works out.

Forget a "universal" spray gun. If you are using automotive paints, you'll want a name brand, air-atomized spray gun intended for that purpose, including a spot gun if you paint a lot of small or individual parts. If you are using house paints, you will want an airless sprayer for the larger jobs and a roller or brush for the smaller ones. A "one-size fits all" product usually doesn't fit anything well.

Rod

Thanks again for the advice. I thought NH was what it stood for, but you never know?

I guess i will review the playing-field for an air-less or go with the rollers...since the corrugated metal sheets are "waved", the undulations would make it more difficult to get an even spray. But, with rollers i would have to go against the rolls to get into the troughs. I am sure i would have to go light on the paint to prevent dripping.

If i have to wire-brush via hand all of the rust spots, how do you say that in spanish? Maybe i can try a couple of test sheets both ways and see what happens.

Good luck with your project and thanks again with your assistance on the boards.

Regards,
tstex