The barrier coats that you refer to are good but you might want to look into the v-c tar from Interlux, striped the bottom of my ’89 32 footer before heading south. I painted the VC Tar barrier coat as recommended: seven 7 coats, then the anti-fouling 2 coats. Went 8 1/2 months around Florida and back, replaced “zinks” once in Florida. Paint looked as good as when I left and Capac readings were always above 0.09 volts. The barrier coat stops the water from reaching the aluminum.

I have used the VC tar before. Used a good primer with a acid etching then zinc chromate. Paint is as good as it gets for the amount of work, and I hope it lasts long. Corrosion will never be stopped by the paint. There are "holidays" in any coating almost and eventually it gets to the hull metal. You have to protect the metal in salt water with “zincs” or, if strictly in fresh water, with magnesium anodes. Do this the paint will take care of its self somewhat. My readings this summer are 1.10 - 1.20 volts where everyone around me in Marinettes are getting only 0.07 or 0.08 volts.

BOTTOM PAINTING PROCEDURE NUMBER ONE:

Water blast old paint off using aircraft striper on remaining paint
Sandblast weld seams with a Sears sandblaster
Sand surface with 120 grit
Wash with acetone
Wash with Alumaprep (phosphoric acid and gylcol) Primewash
Apply eight light coats of Petit 4777/4778 Haze Grey Navy Epoxy
Apply four coats of tin oxide Petit Bottom Paint (No longer legal)
Do Not Pressure Blast Hull at fall haul out! Wash gently anti fouling paint will last 5 years

I bought a 1989 32 sedan in excellent condition except for extensive pitting on her bottom. The previous owners had ignored low meter readings and the anodes were exhausted. They stated very "eruditely", that there as nothing to worry about in fresh water. (Bull!) The survey hauling proved otherwise and significantly reduced the price. The pitting was not very deep, but was extensive. I did a lot of looking around for information on galvanic corrosion, and bought a couple of books, which helped a lot, too.
I decided to do/have done a good job on her bottom:
Removed all thru-hulls.
Stripped bottom, wet sand blasted to bare metal.
Applied Interlux 2-part primer *
Fair-coated the pitting with epoxy fairing compound
Apply eight coats Interlux 2000E 2-part epoxy barrier coat *
Trilux II anti-fouling paint. *

The steps marked with (*) are extremely important. A bronze thru-hull connected electrically to the aluminum hull will make the neatest little battery and the ensuing "whiteheads" will bubble the best paint job, even in fresh water. After a year in the water, the bare magnesium anodes show no corrosion, just turning a little dark. There isn't a mark anywhere on her hull. Capac readings are always in the 1.20 volt range. A lot of work, but I'm beginning to sleep better now after watching it closely for a year.She's berthed 365 days a year so I'll keep you posted on how she hold up.

When I had the 89 surveyed prior to buying her, I was shocked at the condition of her bottom: “Pimples" from stem to stern. Since I was re-doing the bottom all the way to bare metal, I wanted to pull the things and make sure they were isolated in a way that would last a long time. We made delrin spacers and sleeves for the stand-pipes. If yours are isolated, then you are probably fine just refinishing around them. I did a lot of searching and reading prior to putting her in the yard for the bottom. I found a excellent book on corrosion. Most of the books talked a lot of theoretical stuff about the different types (sources) of corrosive action, but dealt mainly with metal fittings on non-metallic hulls. The one I found was much more thorough. As I said, I learned a lot. I'm a typical (read "anal retentive") engineer and don't like it a bit when sinister things are going on where I can't see them. I am beginning to feel more assurance that if cared for properly, these boats will, indeed, outlast their owners. But, as the previous owners of my '89 found out, and I already knew, the "...these things are aluminum and in freshwater they'll last forever.. don't worry" mentality is a big mistake. It will invariably lead to a badly damaged bottom--it just takes awhile.

"...I am quite prepared to have an expensive repainting of the bottom done..."
Are you sure you are dealing with "original" bottom paint? That would have started with several coats of primer plus several layers of anti-fouling. After so many years, that stuff is supposed to wear away -- unless the boat was out of the water for some years.
Is your white spot problem extensive? Lots of them? If so, the white spots are indicative of massive pitting going on below the barrier coat. That means a total bottom job is due.

Every thing I’ve read it all depends on the manufacturers instructions. But one thing that is important is that when one sands to bare aluminum, an etching primer should be applied as soon as possible, for it starts to oxidize immediately. The sooner the primer is applied the better. Trilux 33® is now
being recommended; it contains Cuprous Thiocyanate (meaning it contains copper) which is aluminum compatible.

Some paint mfr's want you to launch within a narrow window but others say you can wait much longer, or that it doesn't matter. When in doubt, ask the mfr. One piece of timing is important, regardless of paint brand: bare aluminum oxidizes, quickly generating a protective coat that will resist paint adhesion (paint won't stick well to oxidized aluminum). Once you sand it down, etch-coat the aluminum quickly, then add the first coat of epoxy barrier coat; that will seal it and prevent oxidation. Then you can take your time applying additional coatings.

The job of bottom painting a boat isn’t a pleasant task and no one wants to do that nasty job again in a year or two.

Here's what Interlux had to say on the situation:
"In regards to your email you will want to apply the Viny-Lux Primewash as soon as you finish blasting it. If you wait a week or longer you will have to get it back to bright metal before you apply the Viny-Lux Primewash. We would recommend that you do it as soon as you get it to bright metal. Do not wait longer than that because oxidation will start to occur, and it may cause poor adhesion with the next coats.
If you have any other questions or comments please feel free to contact us either by email or by phone at 1-800-468-7589 Monday 10:00 AM to 4:00 PM and Tuesday to Friday 9:00 AM to 4:00 PM EST.

The previous owner(s) of my boat, a 1967 Express Sedan, had over painted it a couple of times and the paint was breaking down badly; blisterig and peeling. I tried sanding to the original paint but that took tons of time and was extremely time consuming and very uneven. I have used paint remover to bring it to bare metal and am almost finished with that. The aluminum is in great shape since the boat has been trailered or out of the water most of its life.
My plan is to finish removing the paint from the hull bottom and topside, mechanically sand flat areas or sandblast small areas that would be hard to sand, then treat the bright aluminum. I can see I will have to sand some, treat the sanded area, sand some more, treat etc until it is all sanded and treated. Then I can paint. Still exploring on which brand of paint to use although I have decided on the E-Paint for the bottom. The previous owner used a copper based bottom paint and where there was a breakdown of the underpaint, I got a little surface corrosion but it is not bad at all.

When I re-did my ‘74 I used all Interlux paint products & had great success top side, and also with the Trilux. Top side I had a lot of the removable stuff powder coated. I’m slowly powder coating a lot of the stuff on my ‘89; this winter I had rear deck railing done. The powder coater matched the Sapphire blue really close; way to go for a low maintenance boat. You probably already know that silica sand will make the aluminum look orange peal like. I changed course an finished up using walnut shells for blasting nicer finish.

The marina is going to bottom coat and buff the free board prior to launch. When I looked at the boat before buying it, I noticed a couple of places below water line that were shiny metal. Am I to assume that the marina takes the right steps in prep and prime of these spots before coating the bottom, or should I do this myself? (Hell yes, do it yourself.) Guess I could hand sand the spots and apply a coating. Hate to buy a quart of paint for 2 areas the size of 50 cent pieces. (No choice.)

I myself would touch up the bare spots just to be on the safe side. Not to say marina wouldn’t do it right, but if you want it done right, do it yourself. I’ve used out board / stern drive primer paint spray can kit for similar situations. West Marine sells only two colors: B & W.

From my experiments on coating aluminum, you'd better be really careful. Unless your marina guys are up to speed on treating aluminum--which few are (plastic boats, yes, not necessarily aluminum). Aluminum needs to be sanded, then etched before hitting it with an epoxy barrier coat (followed by anti-fouling paint). The first step is critical or the coating will fall off--and even if you paint the aluminum the second your sander stops turning!
Agreed with the above advise. The important thing for you to do is to look carefully for any missing paint spots; that's the critical thing to do. Small, bare spots that expose the aluminum hull to the water will cause rapid corrosion/ pitting at that point, like a magnifying glass. My hull looked 'a bit rough' before it was sandblasted, and God awful after sanding! Obviously, no one wants a corrosion mess like this secretly 'growing' on their hull! The insidious thing about this is that I had no idea this was happening before the hull was sandblasted to bare metal. You CAN, however, get a hint by looking carefully at any paint blisters and/ or missing paint spots. There were hundreds of them on my hull, with the yellow barrier coat either showing or missing.
BOTTOM PAINTING PROCEDURE NUMBER TWO:
In the Spring of 2004 we redid the bottom of our 32 footer. Below is a practical, ‘real world’ procedure I went through with all of its surprises and difficulties.
The hull was sandblasted to “white metal” with ordinary sand (which does not harm the aluminum). Since I wanted the areas where the jacks go to be done as well, I stripped, sanded and painted these areas first. The stripper required several hits to get most of the bottom paint off. Next comes sanding the bare spots with 120 grit disks to remove the remaining paint, followed by pencil grinding (with an air grinder) any remaining pits to clean out the remaining paint.
I now had four bare metal areas, about 2 foot square, where the pads that will hold up the boat go (after sandblasting) will go. The procedure for painting these four areas is the same as the rest of the bottom:
1. Etch the bare aluminum with Sherman-Williams ‘Dual-Etch’ metal cleaner and conditioner, part # W4-K263 or equivalent. A quart of this nasty stuff is enough for a battleship. Put it in a spray bottle, diluted as specified, and pump spray the bare metal. The etchant has to sit on the bare aluminum for a minimum of 5 minutes. If it dries on you, than you'll have to re-spray, for it must be washed off with water BEFORE it dries. Important! The slippery film left behind, if the above is not followed, will not hold paint..
2. With the surface now etched, see if you have any pits showing (probably). If so, fill them with an epoxy filler such as PC-7 marine epoxy. This stuff takes at least 24 hours to harden enough for sanding. After sanding, re-etch the areas where the sandpaper removed the etchant and washed it off before it dries.
3. The surface was now ready for vinyl wash primer; a self-etching primer that improves the adhesion of the barrier coat. (This material combines etching and primer in one. And it is NOT washed off.) If you can, paint it on with a spray gun for it’s very thin. (It colors the surface a grayish color and needs to dry thoroughly.) Vinyl wash primer has to be over-painted within 4 hours or it will need to be re-applied, so schedule carefully.
4. Put on a minimum of two coats of epoxy barrier coat, allowing each coat to “hand tach dry” before adding the next one. When the last coat has “hand tach dried”, it’s time for the anti-fouling paint.
5. I used E-Paint anti-fouling, which requires a minimum of 4 hours before re-coating. Let that go overnight before adding the last coat.
As you can see, timing is critical to get on all these darn coats of stuff correctly. Finally, it took 4 gallons of barrier coat to do the 32 footer, and 3 gallons of E-Paint.
You talked about multiple pits in your hull. Was your boat in Salt Water? (Briefly.) Mine has many layers of paint on it and has always, as far as I can tell, been in the Great Lakes. Most of the bottom and side paint seems intact with only the cabin top alligatored. Some of the outer bottom layers showed some oxidation and ran off like watercolor when power washed! The sides however held up under 2700 psi of power washing, with only minimal paint removal.

ON SANDBLASTING THE BOTTOM WITHOUT WRECKING THE CAPAC SENSOR:

Let them know that the round thing down there that says "Do not paint" also means "Do not sand blast"! It's a transducer.

I've spent a total of 7 days now working on the bottom paint and the rudders and I don't want to have to do this again for a lot of years! Hopefully next year I can spend 1/2 day sanding and then paint in the same day without all of the sanding of loose paint down to bare aluminum and then going through the 3-4 days of etching, priming and painting.
It's amazing how many days it takes when you need to do 5 coats of primer with a min of 3 hours/max 24 hrs between coats and then 3 coats of Trilux with a min of 12 hours between coats.
(On the claim that you don’t have to etch aluminum before coating it:)
Don't believe them! I ran tests with epoxy barrier coat on bare, cleaned aluminum (you can too). It was obvious that, if you don't etch the aluminum, the paint pulls right off.
(On letting the boat sit a couple of days before etching:)
I STRONGLY advise using a pure etchant FIRST, followed by a vinyl wash primer. You'll need a quart of Sherman-Williams ‘Dual-Etch’ metal cleaner and conditioner, part # W4-K263. (Cheap.) Use a spray bottle and zap the entire bottom with it--twice--then hose the second coat off with water BEFORE it dries. (Let the first coat dry while you hands recover!) This is important, for Aluminum "rusts" instantly after sanding/ blasting, forming a slippery oxide layer that NO paint will adhere to. Trust me on this! You don't want to do the whole $$$ miserable job over next year (or, worst of all, have massive pitting occur where the paint fell off).
I sold a 41' Carver that had been painted 5 years ago when it was pulled; there was very little growth and no blisters. I know the two Expresses on my dock that has not been pulled in 4 years. The meters on their boats show good.
The other interesting thing about the lakes in our area is there are very few boat lifts. If you have to pull you have to rent a crane or trailer to get them out of the water. Also there are a number of aluminum hulled house boats and pontoons that do not have meters and have had no trouble with corrosion with no bottom paint, or at least that is what I have been told.
I gave her 5 coats 2 primer and 3 ablative (2 red and 1 blue). She is still mostly blue. Stern and waterline a bit gooey but the bottom is clean. I am considering using a hard paint if possible when I do haul and paint.

I have the entire hull sanded down to bare metal from the gunwales down (ugh!). I used a 6" random orbit sander with 80 grit discs, after 3 coats of Strypeze. Once I get my corrosion/pitting spots welded (they are inside the hull along the keel), I will finish sanding the topsides then start painting.
I already have bought Interlux prime-wash and underwater primer, still got to get the rest. I intend to hit it again with higher grit discs before I etch with prime-wash. I need to check out this other paint, because I would like to finish painting and then not be in a hurry to finish the rest, as it will take me a while. It's a complete restoration job (as in the boat is totally gutted), so it might take a while
Frankly, since we are NOT talking about a 'plastic' boat here. The only thing that could sabotage you is hull pitting. This is hard to spot with the paint on, but I recommend you crawl under there with a flashlight and knife and investigate any suspicious corrosion spots. If there's any possibility that pitting is evident, then you want to figure in the cost of a complete bottom job (sandblast, etch, epoxy coat, anti-fouling). That can run 4 to 5 grand if you have it all done, so plan accordingly.

BOTTOM PAINTING PROCEDURE NUMBER THREE:

After sand blasting cleaned bottom with 125 psi air.
Using all Interlux products, applied 353 Viny-Lux Primewash, 6 coats of Interprotect 2000 E 3 white & 3 gray alternately and 2 coats of Trilux - II anti fouling; all paints were applied by roller.
Now I have to start on topsides and the rest, which takes a lot more skill. I have very little painting experience but determinate to do a good job. Having pressure washed the boat to remove the chalk on the paint. Now what to do next, fortunately I have a well-equipped heated shop including good spray equipment, for paint I would like to use Interlux Bright side polyurethane maybe the easiest to work with.
The Tempo Marine Zinc Chromate Primer was formulated for out drives & outboard aluminum housings, so I don’t see why it wouldn’t work on small areas of our hulls. When spotting I’ve found the aerosol Zinc Chromate Primer to hold up, but I also apply at least 2 coats Primocon under water primer over the Z/C/P to seal the immediate area before applying A/F.
Anti-fouling, although it’s called a paint, truly isn’t. All in all you must take the same precautions when spotting as you would with any bare aluminum area.
Ablative paints are designed so that with the hull moving thru the water it washes the growth away the faster a boat goes the better it works in keeping the gunk off the bottom.

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Unzinced ships sink at slips.