What are these black spots?

[Pierce89]

Does anybody know what these black spots are on a 1960 aluminum boat? I am removing white paint and the primer that I think is the original 61 year old paint from the hull with an orbital sander and when get to these black spots it's like gunpowder coming off. They're flat with the rest of the hull. Did aluminum have discolorations back then?

 

[dingbat]

Chemically induced Oxidation….a number of different contaminants can cause this but it’s typically bad surface prep in these kinds of situations.

Need to completely remove the oxidation prior to refinishing

 

[racerone]

My guess is water getting under the paint.----Corrosion occurred.----Aluminum behaves the same over all the years.

 

[Pierce89]

Thanks for the replies, I was hesitant to continue sanding that spot but I assume I need to continue until it's the same color as the rest of the aluminum. 80 grit sandpaper is taking awhile to sand this entire boat but hopefully I'm not taking much aluminum off with it.

 

[dingbat]

Thanks for the replies, I was hesitant to continue sanding that spot but I assume I need to continue until it's the same color as the rest of the aluminum. 80 grit sandpaper is taking awhile to sand this entire boat but hopefully I'm not taking much aluminum off with it.

Is the staining just on the surface of the aluminum or mixed in with the paint?

Black staining on aluminum is typically induced by low PH condition. If the coloration has leached into the paint you need to remove the surrounding paint as well. If it's restricted to the base metal, I would try using something mildly acidic.

White vinegar even lemon juice works pretty well on aluminum. Soak a rag and let it sit on the stain. If that doesn't work try something a little stronger. Pontoon cleaner?

 

[Pierce89]

The 2nd picture shows better, the paint comes right off and leaves the black stain but I was hesitant to sand the black part anymore to match the bright aluminum shade of the rest of the hull. I wasn't sure what it was and didn't want to sand through the hull! The white paint in that picture is around a rivet I'll get later with a different tool.

The white paint in the first picture can be removed and it will leave that circle of black gunpowder dust behind. Is there regular aluminum behind it if I continue sanding?

On an sidenote and one that is always on my mind but I cannot find an answer. Can you help please? I am restoring an 18ft old aluminum boat that will be used in saltwater. When I add a deck and seal the edges to gunnels so all saltwater runs along deck to bilge, what happens when a rivet begins to leak again? Let's say a rivet begins leaking in the bow and every rib has 3 limber holes in it. Most saltwater will run to bilge and I can get it out, but what about any saltwater that gets trapped? I park the boat and can't hose under the deck, and a little saltwater sits against a rib or something.

Or hypothetically if a bait bucket full of saltwater falls, water flows into bilge, a wave pushes bow down immediately and the water enters below deck through the aft limber holes. The majority will end up coming back to bilge when the boat settles, but now there's still saltwater that's been under deck and not all 100% of it will exit . Am I overthinking?!

 

[dingbat]

The 2nd picture shows better, the paint comes right off and leaves the black stain but I was hesitant to sand the black part anymore to match the bright aluminum shade of the rest of the hull. I wasn't sure what it was and didn't want to sand through the hull! The white paint in that picture is around a rivet I'll get later with a different tool.

The white paint in the first picture can be removed and it will leave that circle of black gunpowder dust behind. Is there regular aluminum behind it if I continue sanding?

On an sidenote and one that is always on my mind but I cannot find an answer. Can you help please? I am restoring an 18ft old aluminum boat that will be used in saltwater. When I add a deck and seal the edges to gunnels so all saltwater runs along deck to bilge, what happens when a rivet begins to leak again? Let's say a rivet begins leaking in the bow and every rib has 3 limber holes in it. Most saltwater will run to bilge and I can get it out, but what about any saltwater that gets trapped? I park the boat and can't hose under the deck, and a little saltwater sits against a rib or something.

Or hypothetically if a bait bucket full of saltwater falls, water flows into bilge, a wave pushes bow down immediately and the water enters below deck through the aft limber holes. The majority will end up coming back to bilge when the boat settles, but now there's still saltwater that's been under deck and not all 100% of it will exit . Am I overthinking?!

Well…hate to be the bearer of bad news but aluminum doesn’t hold up well to saltwater.

Dissimilar metals corrosion, stagnant (trapped) water (no O2 to replenish Al oxide) resulting drop in PH and crevice corrosion (at the rivits) will be your demise.

Anodize and paint helps for a while but sooner later pitting begins. Powder coating of hard and T tops to help slow the corrosion is now pretty common.

If I don’t hose down my hardtop after each trip it will “dust” (white powder) and start to pit in a matter of days. The gimbals on my trolling rods are pitted and I’m religious rinsing them off after use.

You could load up on “zincs” and numerous counter measures in an effort to slows the effects but it’s a loosing battle IMHO

 

[Pierce89]

I understand just couldn't pass up on such a cool hull. I'll paint it and not leave it in saltwater more than 24hrs and hopefully get some life out of her for a few years. I don't see any corrosion unless those spots in the images are very bad. Aluminum boats stay in wetslips in the wet coast and Australia year-round so hopefully trailering this one and hosing it down after each trip will give some great years.

Hopefully the sun in Florida will heat up the aluminum and evaporate any saltwater under deck fairly quickly after an outing and with anodes it won't destroy under the deck too quickly.

 

[dingbat]

Aluminum boats stay in wetslips in the wet coast and Australia year-round so hopefully trailering this one and hosing it down after each trip will give some great years.

Properly maintained, welded aluminum (heavy) boats are one thing. Riveted is an entirely different beast.

Keep an eye on the rivets… that’s where it is where the corrosion will manifest

Evaporating saltwater leaves the corrosive “salts” behind.

Decking over the bottom is not a good idea unless it can be removed for frequent cleaning and inspection if you don’t want your first sign of problems being wet feet

 

[Lectro88]

Properly maintained, welded aluminum (heavy) boats are one thing. Riveted is an entirely different beast.

Keep an eye on the rivets… that’s where it is where the corrosion will manifest

Evaporating saltwater leaves the corrosive “salts” behind.

Decking over the bottom is not a good idea unless it can be removed for frequent cleaning and inspection if you don’t want your first sign of problems being wet feet

Evaporating saltwater leaves the corrosive “salts” behind. ------ ** Nailed it..**

Then you only have the 1000's of rivets-- 1 at a time.

Prayer, hope and wishful thinking... with some lowered expectations may ease the day of disappointment to eventually come.

Just enjoy it and do the best with the life it gives.
I have battled aluminum, thick aluminum and elements away from salt water.
I would dread the thoughts of a thin aluminum hull and saltwater... knowing my luck.(for me)

a twisted thought..
Have you ever had salt in a open wound,. it burns and stings.
Salt does that to aluminum.. all the time-- 24/7

 

[zool]

I would just degrease it, sand it, paint it, and go boating.....if i wanted it to last longer, i would pay more for better paint.

 

[Pierce89]

Yeah that's the plan. A few hundred dollars in paint, alot of plywood, and some CSM/poly resin over the plywood and hopefully I get some years out of it. For a boat that will be trailered in and out of saltwater, is it advisable to paint the bottom or strip the paint and leave it bare aluminum so it can create that white dust layer that protects it? I was planning on using Total Boat products for the sides of boat and top but not sure about the bottom paint or to leave it bare.

 

[zool]

Yeah that's the plan. A few hundred dollars in paint, alot of plywood, and some CSM/poly resin over the plywood and hopefully I get some years out of it. For a boat that will be trailered in and out of saltwater, is it advisable to paint the bottom or strip the paint and leave it bare aluminum so it can create that white dust layer that protects it? I was planning on using Total Boat products for the sides of boat and top but not sure about the bottom paint or to leave it bare.

TB makes an aluminum hull epoxy barrier coat primer that would protect it from further corrosion, they also make an etching wash for prep of bare aluminum before the epoxy. Its brush, roller or spray, then paint what you want on it.

https://www.totalboat.com/product/aluminum-boat-barrier-coat-primer/