PAINTING MILD STEEL for boat trailer guide base?

I made this boat trailer guide:
http://igor.chudov.com/projects/Trailer-Guide/
I need it because we frequently go boating on a river with very strong
current, which makes keeping the boat aligned over the trailer difficult during retrieval.
The base is made of 3/8" thick hot rolled mild steel. The bolts are half inch bolts.
How do I paint it to prevent corrosion?
thanks
i
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Ignoramus26696 wrote:

Best solution is hot-dip galvanizing. Watch out for minimum order, though -- my galvanizer charges $75 to do one toothpick up to about 200 pounds. I have never tried cold galvanizing. Some hardy souls on this NG might consider melting up some zinc in a furnace and doing DIY hot-dip galvanizing on this part because it's small enough to maybe fit into a crucible. But I wouldn't.
If you're just going to paint it, disassemble it and clean it with solvent (paint thinner) and blow it dry, then give the non-galvanized parts (not counting the bolts, those are stainless, right?) a coat of ruddy red primer. This coat should be perfect, no little pinholes. Maybe 2 coats. Then topcoat it with any quality enamel to the color you like -- it's the red primer that does most of the antirust work.
A part this small wouldn't be too much trouble to derust and repaint every decade or so. Navy ships are made of steel. Perhaps you've heard of the old Navy aphorism: paint the whole ship one end to the other, then start over again. They just use good paint and a whole lot of it.
Grant
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Ignoramus26696 wrote:

Personally I would degrease it with white spirit, apply a coat of red oxide primer, then two coats of a decent quality topcoat such as Rust-Oleum or Hammerite. I think both Rust-Oleum and Hammerite say you can apply them without a primer, but I don't. I find they adhere better (especially to aluminium and zinc) if you use a primer. I've also used something called "pylon paint" in the past, but I don't know its official trade name as I didn't buy it myself. It's literally the stuff the electricity company use to paint pylons. I think it might also be known as "cold galvanising paint". This is very weather resistant and you can use very thick coats, but the surface finish is rough and it looks ugly. Good if you don't view the paintwork too closely! Whatever you choose, remember to repaint the item as soon as you see rust. This is where most people go wrong.
Best wishes,
Chris
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Thanks, Christopher and Grant. I will paint them using rustoleum primer and "hammered finish" topcoat. I will degrease the parts prior to applying primer. Now I have to make a second copy of this item, for the starboard side of the boat.
i
On Sun, 26 Jun 2005 15:59:50 +0000 (UTC), Christopher Tidy

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You'll most likely need to deal with rust on a regular basis (maybe annually), as parts that are dipped in lake water then stored outside year 'round are difficult to make rust-proof (particularly when they're bolted together and subject to shock loads).
I'd avoid using the galvanized pipe if the other parts are not also galvanized. Primers for galvanized steel usually aren't the same as for untreated steel.
If these guide bases? are to be extended upward vertically, the threaded pipe ends will be relatively weak. Ideally, the vertical guides should be one piece (they may bend, but not snap off when you aren't expecting it).
Because of the complex mating surfaces and the mechanical tension of the hardware, the assembly should probably be dipped (or thoroughly coated) in something thick that will have good adhesion. I haven't tried it myself, but there are a lot of positive comments about the (aerosol or brush on) bedliner coating products. That might be something that's capable of being applied heavily in multiple coats. I'm not familiar with what prep work/primer coat would be suitable for those products. I assume they are to be applied over a good quality paint job.
There are a couple of ways a protective coating works. If the steel is completely clean and is provided with an appropriate surface treatment, the correct primer coat will adhere properly. When the primer coat is properly prepared, the paint provides the protection. Many primers provide very little protection from the weather or severe environments/conditions.
For steel that will be outdoors, I clean the metal thoroughly, then treat it with a phosphoric etch solution applied with steel wool or scuffing pads, with a vigorous scuffing action to get a good etch and provide a surface that isn't slick. Primer generally won't adhere properly to a slick surface (it may stay there for a short time though). After about 20 minutes of being continuously wet with etch, the part is rinsed with fresh/clean water and dried as quickly as possible. Clean compressed air, a heat gun, radiant heat or other forced drying method that doesn't contaminate the surface. The primer should go on as soon as possible after the part is completely dry, and even better if the steel is still warm. If spraying from a gun, give the primer a final stir, and start applying it. Repeat after a waiting period determined from the manufacturer's guidelines. It's usually difficult to determine how much primer is applied when it comes from a spray can because the stuff is really thin (even if it's properly shaken). There is a lot of solvent, and only a minimal amount of solids.
After the primer is dry, an appropriate finish coat(ing) can be applied. Some primers require scuffing before painting, for maximum adhesion of the finish coat. Forced drying of enamels will often improve their durability. This can be accomplished by placing the parts in sunlight for the day, on a sunny day, and turning them over a few times. Dark colors will absorb faster than white or light colors. Most paints aren't intended to be applied in excessively heavy coats, the results will most likely be that the drying action will cause the paint coating to fail.
Proper preparation and application through the entire process insures maximum durability.
WB ...............

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I'd simpy paint the whole thing with Rustoleum every two years, but more importantly cover it with indoor/outdoor carpet so that it doesn't damage the hull of the boat.
Harry C.
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Thanks to all. I cleaned up the pieces, applied a rustoleum primer, and applied one coat of rustoleum metallic hammered looking paint. I will apply another coat soon.
This setup will be suspended in the air 99.999% of time, and will be dunked into freshwater a few times a year at best (quite possibly very little next year). Given that it is made of 3/8" of steel, it is unlikely to be damaged structurally by rust before I replace this boat or I die.
i
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Ignoramus26696 wrote:

This started me thinking. You're most likely right, but as I said to Tom Q. earlier, engineering offers plenty of surprises. You get some pretty weird-ass corrosion sometimes. I looked at a few websites on pitting corrosion and found figures quoted for mild steel corrosion in harsh atmospheres of several mm/year. Unfortunately I couldn't find a figure for pitting corrosion in a pretty regular outdoor, damp, non-salty atmosphere. Having watched stuff corrode myself, I reckon leaving something outside with a bit of missing paint might corrode at 0.2 mm/year if pitting started. At that rate you'd have 48 years if you were unlucky, so I reckon you're safe. But I saw an engine which had been left outside with the cylinder head removed for maybe 10 or 15 years at a boatyard on the coast. There was a hole corroded through a piston maybe 10 mm thick. It makes you think.
You can tell I'm bored tonight...
Chris
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On Mon, 27 Jun 2005 01:04:39 +0000 (UTC), Christopher Tidy

Yes, I agree that it is unpredictable. My old trailer rusted to the point that it nearly fell apart (I am a very lucky guy).
i
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Why not have it powder coated?
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HF sells a powdercoating setup that can be purchased for less than $60 at times, and a 1/2 lb can of powdercoat paint add another $10. You can use a old electric oven from the thrift store and for less than $100 total do lots of smaller stuff. If you look on eBay you can get powdercoat paint for $10 - 12 a lbs.
I do lots of projects with my small powdercoat system, I prefer it to spray painting any day. B.G.
Mike wrote:

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Thanks guys. While I appreciate the advice, the items have already been painted with rustoleum and primer.
i
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