Tape on alloy mast corrosion risk?

[RoyB]

Any problem trying to seal the mast through deck area with some 3" wide vinyl heavy duty special adhesive 3M tape that I have? (applied under the mast boot).

Reason I ask is the company that manufactured our hydraulic backstay cylinder says not to tape anything to it as the tape will retain salt water under itself on the cylinder and cause corrosion.

Mast has a boot but some slight amount of water still penetrates under the boot and into the cabin.

[Renoir]

Stainless steel mast steps were installed over 25 years ago using 3M mylar tape to insulate against corrosion. This worked well because the mast was well painted with the usual green undercoating (a chromate?) and a good paint over that (not linear polyurethane although that would have been good as well, I believe).

The interface between the tape and the paint at the edges of the tape do not provide any egress into the aluminum even with salt water all over the place as long as the coatings on the aluminum remain intact. Therefore, tape against any metal may provide a collection at the edes of salt water that could form an electrical couple yet when tape is against an insulator (a non-conducting paint) there should be no problem.

[Douglas]

I agree with what Renoir replied , but I am at a loss to understand, as to how corrosion begins under a proper undercoat and then top coat of paint on an aluminum spar.

One person I talked to said, that even a finger print, or , a high humidity condition , on the preped aluminum surface, before the undercoat is applied, could and can , cause corrosion under a painted surface on an aluminum spar ???

Could it be that the undercoat and top coat of paint is porous enough to allow ingress of moisture which accelerates a corrosion problem and then is evidenced on the surface ?

Another question is, if UHMW tape is applied over a potential corrosion problem spot , could that tape reduce the corrosion potential at that spot ? Especially at the mast butt to mast step area ?

On my Forespar mast , I found an enlargening corrosion spot under the LPU painted surface just above the dining table inside the cabin. On closer inspection , I found a horizontal joint line in the sailslide groove, which is where the corrision seemed to start from .

From this inspection , I assume that Forespar, welded , aluminum mast sections together, to make a longer stick , and that the joint itself was contaminated and allowed the corrosion to break out under the paint job on top of it.

I had the mast carefully, glass bead blasted with un-recycled beads, then the Alondine prep, before the Algrip primer and top coat, but after 2 years, that surface corrosion began to show through, under the paint, and enlarged at that suspected joint !

I like it, that Brion Toss says, " try to build in a reserve of neglect" , when refering to rigging practices , but how can we best do this, when there is sooo much to know and look out for ???? I mean ,,,, all we wanted is to go sailing ,,,, right ????

Does anyone know if , powder coating is a better option to fight against corrosion than proper surface painting on a mast and boom ?

Douglas ,,, frustrated in a corrosive world !