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Old 24-04-2017, 05:02   #1
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Sandblasting Aluminum hull and new plastic thru-hulls

Hi guys!

As part of my refit I plan to put a new thru-hulls (from Trudesign).
I also plan to do a full new below waterline painting of aluminum hull - so sandblasting, priming, epoxy, paint

Question is - when should I install the new thru-hulls? Before or after sandblasting?

Any advice are welcome.
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Old 24-04-2017, 06:50   #2
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Re: Sandblasting Aluminum hull and new plastic thru-hulls

After.
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Old 24-04-2017, 07:09   #3
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Re: Sandblasting Aluminum hull and new plastic thru-hulls

Could you explain why after would be better?
How would you properly clean the place before installing? Till the raw metal or till epoxy layer?
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Old 24-04-2017, 07:18   #4
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Re: Sandblasting Aluminum hull and new plastic thru-hulls

The blasted surface will grip sealant very well. You'll get better seals and appearance than mounting them on top of old paint.
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Old 24-04-2017, 07:32   #5
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Re: Sandblasting Aluminum hull and new plastic thru-hulls

For our transducer, we cleaned the surrounding surface, continued the epoxy barrier coat fully encompassing the hole, and caulked in place.

My thinking, while the caulk bonds to the aluminum well, any break in the barrier coat is a potential spot for corrosion to set in.

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Old 24-04-2017, 07:41   #6
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Re: Sandblasting Aluminum hull and new plastic thru-hulls

Assuming that your through hulls aren't aluminum or plastic, you need to fully electrcally isolate them from the hull. Which means applying your coatings, including in the vicinity of any & all though hulls first. And then installing your through hulls, & other skin fittings, using isolators. To do otherwise is to build a "battery", where the aluminum hull near the fitting acts as the anode. Which is definitely a losing proposition. Meaning that the differences in electrical potential between the metals when immersed in an electrolyte (seawater) will cause the aluminum to be eaten away just like a zinc in a "hot" harbor.
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Old 24-04-2017, 07:43   #7
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Re: Sandblasting Aluminum hull and new plastic thru-hulls

Matt I think that's as good or better idea than mine I'm discounting corrosion because it's under the waterline and I'm assuming good anodes. While everything above the waterline on my steel boat corrodes continuously, even bare steel underwater is corrosion free. Of course, any metal boat takes a bunch more anodes than a FG hull.
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Old 24-04-2017, 07:49   #8
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Re: Sandblasting Aluminum hull and new plastic thru-hulls

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Originally Posted by UNCIVILIZED View Post
Assuming that your through hulls aren't aluminum or plastic, you need to fully electrcally isolate them from the hull. Which means applying your coatings, including in the vicinity of any & all though hulls first. And then installing your through hulls, & other skin fittings, using isolators. To do otherwise is to build a "battery", where the aluminum hull near the fitting acts as the anode. Which is definitely a losing proposition. Meaning that the differences in electrical potential between the metals when immersed in an electrolyte (seawater) will cause the aluminum to be eaten away just like a zinc in a "hot" harbor.

The thruhulls he mentioned in the OP are plastic.

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Old 24-04-2017, 08:29   #9
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Re: Sandblasting Aluminum hull and new plastic thru-hulls

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The thruhulls he mentioned in the OP are plastic.

Matt
Sorry, some evil Gnomes must have switched me to decaf this morning. That said, it' still worth fully sealing as much of the hull as is possible, prior to putting in the through hulls. That way you're not just depending on the sealant around the fitting to act as a barrier coat. Instead, rather you have the barrier coat(s), plus the sealant. And the gaps in the boat's skin coatings are much, much smaller.
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Old 24-04-2017, 08:57   #10
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Re: Sandblasting Aluminum hull and new plastic thru-hulls

Glas or soda blasting NOT sand! sand is way to coarse for aluminium: peace:
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Old 24-04-2017, 09:02   #11
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Re: Sandblasting Aluminum hull and new plastic thru-hulls

I know that I'm bringing up something you didn't ask about, but thru hulls are kind of a BIG deal, because their failure is SO catastrophic!!!

I did this a few years ago, looked into the plastic, but opted for the most robust, strong, bullet proof system I could find. When you torque the gate valve on a through hull that's full of critters with hard shells on them, I want the whole system stronger than any plastic! This is not a subject to cheap out on.

For this reason, I would like to offer you a very comprehensive article on this subject. NO comparison in strength between these thick bronze fixtures and any plastic one: Replacing Thru-Hulls and Seacocks Photo Gallery by Compass Marine How To at pbase.com
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Old 24-04-2017, 09:15   #12
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Re: Sandblasting Aluminum hull and new plastic thru-hulls

Bronze is a no go on Aluminium
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Old 24-04-2017, 09:57   #13
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Re: Sandblasting Aluminum hull and new plastic thru-hulls

paint it all with the through hulls out that way the cross section of the hole is coated then install with bedding compound.


I prefer marelon for aluminum
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Old 24-04-2017, 10:09   #14
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Re: Sandblasting Aluminum hull and new plastic thru-hulls

I wouldn't use gate valves on a boat. I know bronze, copper and stainless steel make problems for aluminum hulls, but I see S/S stanchions isolated in their holders, so why can't a bronze seacock be electrically isolated from an aluminum hull?
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Old 24-04-2017, 12:01   #15
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Re: Sandblasting Aluminum hull and new plastic thru-hulls

I strongly agree with soerencarlsen ! Sand is ablative to Aluminum.Soda or glass is a must.Additionally,those materials will aneal aluminum and make it stronger.
As to seacocks- Delrin for aluminum.FWIW.


All the Best.
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