Repainting a new boat

[H20fwler]

I bought a boat several months back and the paint is flaking off in places. It does not look like the aluminum was sanded prior to painting. The builder said he would repaint it if I brought it to him (10 hr drive) I am thinking about just doing it myself.

Should I take the boat and have it blasted then repaint it? I know I can do a better job of painting it but I don't have a way to remove the old paint. it is .190 gauge aluminum.

[longliner45]

10hr drive or 20hrs in labor and cost?hmmmmmmm

[safewalrus]

OK so longliner has a point (he always has, some very good ones too) but look at it this way if the builder is willing to repaint it he knows he made a mistake the first time, so what's to prevent him doing it again? especially if it's for free? Think I'd do a little more research on the removal etc of the poor paint and redo it myself (so Ok you've got no one to blame but at least you know the standard of the painter, and the paint! you know it's been done properly!)

Just a thought

[Jimbo1490]

The adhesion of the paint job is obviously suspect. There is nothing you can put on top of a coating that is not adhered well to make it adhere well. It must be removed and applied properly. It should be stripped and refinished.

Painting aluminum properly is kind of a big deal compared to steel or composite. It is a reactive metal. It is always corroding. It just happens that its oxide is very hard and stable and prevents further corrosion, which makes people think that it's not a reactive metal, which in fact, it is.

Trouble is, you can't get lasting paint adhesion on aluminum's oxide layer. You have to remove it. As soon as you do it immediately tries to form again. So you must stabilize it somehow long enough to get paint on it. This process is called passivation.

Phosphoric/hydrofluoric acid solutions are used to take off the oxide and 'brighten' the metal, then a mixture of chromic acid and metallic salts (Alodine) is used to passivate the surface. Now you can get the paint to stick permanently.

There are 'shortcut' products such as etching primers and 'wash' primers which work OK, but they are no where NEAR as durable or prevent corrosion/paint sloughing as long as the real deal, which is acid wash and Alodine. Unless the item in question is (a) easy to strip and refinish every few years or (b) not going to be in your posession very long, then it's much better to use the etch/Alodine process than an etching primer since it's the labor of refinishing that is the major expense; the less frequently you must refinish, the better.

I've done this process on big stuff (airplanes) thousands of times, so it's no big deal for me. But most people who try it for the first time say it is a miserable, laborious, messy job. The etch burns your skin and the Alodine is full of hexavalent chromium, a toxic heavy metal compound

Something to consider.

Jimbo

[Jimbo1490]

BTW sanding is of NO VALUE in promoting adhesion on clean, uncorroded aluminum. If there is heavy white powdery oxide present, then sanding is one method of removing that, but it is only a first step. You still have to brighten then passivate. Sanding does nothing to passivate the aluminum.

Jimbo

[Bob S.]

Jimbo - if you don't mind - how is alodine applied and where is it typically purchased?

[Jimbo1490]

Most auto paint dealers will have it. In the Dupont line the part numbers are 225S for the acid cleaner and 226S for the Alodine. In the PPG line it's DX503 and DX533. Lewis Marine carries the actual Alodine brand, now owned by Henkel. The numbers are Alodine 1201 and Alumiprep 33. Aircraft Spruce and Specialty carries the Henkel line as well. I buy mine in dry form from Eldorado chemical where they call it Doradokote.

Jimbo

[Bob S.]

Thanks!

[mastcolin]

Originally Posted by Jimbo1490

BTW sanding is of NO VALUE in promoting adhesion on clean, uncorroded aluminum. If there is heavy white powdery oxide present, then sanding is one method of removing that, but it is only a first step. You still have to brighten then passivate. Sanding does nothing to passivate the aluminum.

Jimbo

Mmmm, not sure about your comment re: 'no value'. Try physical key. That's a fine reason to abrade surface.

Alodine/chemicals are mean to use. Corrosive, you have to dispose of waste through disposal service (and you need loads of water to rinse) - not down drains. And it stinks!

I work for company that regularly paint aluminium yachts, 10m -90m (eg Jongert, Royal Huismann etc) . We alodine interiors but exteriors are abraded. And we stick on filler. Adhesion is great.

Ideal is both sanding and chemical etch.

All yacht paint companies say 'sand' aluminium for best results.

[Jimbo1490]

Most of the time when you repaint especially a yacht, there is going to be corrosion, often quite a bit of corrosion. Also, thousands of paint flecks (after chemical stripping) and numerous nicks and scratches are also best addressed by sanding. Then, as I stated, sanding has a use. But this thread is about a NEW boat with failed adhesion. Sanding is of no value here. Physical key is useless in promoting adhesion when painting aluminum. Our own panel testing proved this to me beyond any doubt. The test panels with the best adhesion had a perfectly smooth factory finish and the 'darkest' Alodine coat. The sanded /Alodined panels did not have impressive adhesion. Sanded alone without etch/Alodine failed right away in that paint could be induced to break adhesion with a tape pull-off test.

Sanding is of course the best way to get the surface smooth and free of defects prior to painting, but that's all.

Jimbo

[mastcolin]

Jimbo

You're not wrong when you say a chemical etch can give good adhesion. However it is essential it is carried out very thoroughly, both stages, well 4 if you include rinsing, with rapid overcoat on clean surface. Do it wrong and it is recipe for disaster.

I don't doubt your test panel looked good. For how long did you test it for? Did you scratch it, leave in damp/dry cycle, UV? and check undercut etc?

The Awlgrip book says abrade, International say abrade. My experience - 8 years in yacht lab with International, 4 years as tech service would say it will peel off in sheets with time if unabraded - and yes, perhaps in main due to poor etching. But every job is balance of effort vs risk/benefit. It's not risk I'd be willing to take, especially as you can quickly abrade as opposed to all the hassle with corrosives liquids then disposal.

As I say. We paint Jongerts, Amels and Huismans amongst others. All abraded then cleaned some with the chemical cleaner.


ps depend on your primer as well obviously. 545 is great undercoat. It sucks as primer on metals. Not every epoxy that works great on steel works great on alloy.

Happy shooting besides.

[Jimbo1490]

I work with vehicles that cannot, or at least should not be abraded, which is aircraft. We first of all don't have enough skin thickness to keep abrading every paint cycle. Even a 747 has only ~.063" fuselage skins. GA airplanes are much thinner than that. Secondly, aircraft are ALWAYS coated with either Alclad or anodized from the factory and should therefore never be abraded overall lest the anit-corrosive coating be removed.

No duty, including marine, taxes a paint's adhesion more than jet aircraft. The thermal shock a jet gets on every flight is something that is never encountered in any other vehicle. Hitting rain at 300+ mph is even worse. In fact they even invented a paint stripping method based on the observation of how difficult it is to keep paint on leading edges of jets that fly through rain regularly (or even once ).

Nevertheless, we manage to acheive OUTSTANDING lasting adhesion on aluminum using etch and Alodine. Many commercial aircraft paint shops do sand every aircraft, though this is not done to promote adjesion, and I have yet to hear of any shop that claims that it is done for this purpose. Instead, it is done to get rid of the surface imperfections that usually remain after chemical stripping, mostly small flecks of unstripped paint, primer and dent fillers and corrosion. After that, the skin is still etched and Alodined. But this sanding is considered a dirty little shortcut in the aviation industry. Nobody in that industry believes that it promotes adhesion; it is just the least laborious 'quick and dirty' method of smoothing out the skins for primer and topcoat.

I have been working on a contract with a major 'upstart' carrier with blue colored airplanes since 2000, so have had opportunity to work on aircraft that were painted at the factory. NONE of these aircraft were sanded; the smooth factory skins were etched and Alodined, then primed and topcoated. We strip small areas of each one of these aircraft to install certaing options, and the skin is always smooth and unsanded underneath the factory paint. There has not been even ONE aircraft with paint flaking off, now after over 6 years of hard service. If the factories like Boeing and Airbus thought that sanding would improve adhesion, they might do it. But they don't. They don't have to worry about paint flecks, bondo or corrosion either since they work with new stuff.

Occasionally an aircraft will strip so clean that even a 'shortcut' shop does not bother to sand. Once in a while, this happens with an aircraft that has never been sanded previously, and it strips to a beautifully polished surface. We then etch and Alodine as usual and do not notice even the slightest loss in adhesion in painting over this ultra smooth texture. Every airplane gets quite a bit of striping and graphics, which affords ample opportunity to locate poorly adhered areas when tape is removed (usually unceremoniuosly ripped ) from stripe/graphic areas.

Which brings us back to the thread, which is how to re-paint an otherwise NEW boat where the original paint coating is suffering adhesion failure. Step one is to get the bad paint coat off. If you can do this with chemical stripper, you'll save a lot of labor. If not, sand it off down to bare metal. If you can't or don't want to strip then sanding is the choice that is left. Just don't mistakenly imagine that this sanding will help your adhesion in any meaningful, lasting way; it will not. You must etch and Alodine to get that.

Alodine lasts ~14 years, BTW. After that, it breaks down by itself and corrosion begins to take hold.

Easiest to use most versatile bare metal primer is Mil-P-23377 yellow epoxy polyamide. All of them (many vendors make it) give you three days to 'hot coat' without sanding; some give you 5 days! I recently painted one during a cool snap more than 7 days after priming with no apparent loss of adhesion using a metal tape rip-off test. The Awl-Grip 'non-sanding' yellow primer meets this spec.

Jimbo