Zinc contact points- sand clean?

[boatdude100]

So I have a corrosion problem SX-A
I assumed I should sand clean the contact points on stern drive and zinc (if it has been used) to give it good electrical contact. However, I believe these "contact points" (around the bolts) are painted over by Volvo. Perhaps the bolts themselves serve as the electrical conductors?
Should I sand for a "clean contact" with the sterndrive?

 

[tphoyt]

I wouldn’t sand myself. The zincs are sacrificial because they are less resistance to the electrolysis in the water than other metals and are essentially attacked first.
JMO

 

[Horigan]

I sand the small raised contact area around bolt holes to improve conductivity to the drive/transom. Yes the bolts provide most of the connection.

 

[Donald0039]

I sand the small raised contact area around bolt holes to improve conductivity to the drive/transom. Yes the bolts provide most of the connection.

I am not sure the SS bolt provides the connection. I think it's the zinc touching the aluminum of the outdrive. The bolt just holds then in contact. SS is not a great conductor of electricity.

 

[boatdude100]

I found it strange the contacts were painted. any one else believe in sanding them to improve contact?

 

[dingbat]

I found it strange the contacts were painted. any one else believe in sanding them to improve contact?

The mounting bolt is the contact point.

Where talking molecular contact, not electrical. Sanding the protective barrier (paint) from the surrounding area does nothing except expose bare metal to corrosion

When was the last time you “sanded” the surface of the “zinc”?

They oxidize over time and loose their ability to protect.

Zinc = saltwater only
Aluminum = brackish and fresh water
Magnesium = fresh water

 

[boatdude100]

Does sanding the surface of a zinc "restore" it if its not badly corroded then?

 

[dingbat]

Does sanding the surface of a zinc "restore" it if its not badly corroded then?

All “zincs” oxidize becoming less and less effective over time.
Sanding, wire brush is better, will restore efficiency, but if there is any erosion or pitting, (looks like sponge) I just replace them

What water type and anode material is in use?

 

[boatdude100]

fresh water, stays in water all summer. put on magnesium Zinc last year- now its pitted. its from
from boatzincs.com

 

[dingbat]

fresh water, stays in water all summer. put on magnesium Zinc last year- now its pitted. its from from boatzincs.com

Fresh water (chemically pure) is becoming less and less common. May have to bump up to aluminum if magnesium gets eaten up too quickly.

If you’re in a marina, your most likely the victim of stray current. Otherwise, a bad ground or a current loop on the boat will accelerate the corrosion.

Check battery and engine ground connection(s) for corrosion and or damage.

 

[Horigan]

All “zincs” oxidize becoming less and less effective over time.
Sanding, wire brush is better, will restore efficiency, ...

I've read that wire brushing is not good for a sacrificial anode as it imparts some steel onto the surface reducing its effectiveness. Rough sandpaper is what I use.