Stanchion Base Bedding

I recently pulled a stanchion base on my Swan 48.   The base was on a +/- 1/8" rubber gasket and the area below is half teak deck and half a portion of the aluminum toerail.   The latter is blistered and has quite a lot of corrosion.

What should I use to build up where the aluminum corroded away and what type of sealant and/or gasket material should I use?

Also, the nuts for the stanchion base bolts look to be glassed into the underside of the deck.   Some of those are now loose and need to be replaced.   There were no washers between the nuts and the underside of the deck so I'm guessing there's a backing plate sandwiched between the top and bottom of the deck layers.  Is that correct?  Should I add some oversized washers?

Area Below Stanchion Base

Dear Larry
It appears there has been a void where water can stand, and this has caused these problems. Here a proposal.
Clean the aluminum to bare metal (do not use steel tools), and cover the affected area with zinc chromate, for example Duralac, and while still tacky apply pressure with a plane object so the surface will be smooth and level with the toe rail horizontal flange, and fills the corroded recess in the rail completely. Zinc chromate will prevent further corrosion.
The suggestion is to remove the damaged teak under the stanchion base, and put in a filler piece with the same thickness as the rail flange. The filler piece should be about 1/8" smaller than the recess, so there can be a bead of mastic all around it, and the upper surface made level with the rail flange. The filler piece could be G-plate. Also the round isolation washer with some 1/8" thickness needed under the base could be of this material. Water should be able to pass freely outboard of the washer, snip off the edge if necessary.

Take care to countersink the bolt holes both in the washer and the spacer and put mastic there when assembling for preventing leaks.
Guess that there is one bolt hole through the toe rail, the recommendation is to make this hole slightly bigger so that the bolt does not touch the aluminum, and put a plastic sleeve around the bolt, or several layers of electrician's tape.
3M5200 mastic would be fine.

There are no hidden backing plates, suggest one is added on the underside, you could use G-plate here also, same thickness and a little more diameter than the filler piece above. Use washers under the nuts.

What about the other stanchion bases, could there be similar problems?
Kind regards
Lars

Many thanks Lars.

There was a black rubber gasket between the stainless stanchion base and the teak and aluminum deck. Should that gasket be replaced and, if so, with what material?

Also I find 5200 can be difficult to remove and sometimes gets brittle. Can you suggest something else for the mastic?

Larry

Dear Larry
The suggestion is not to use the rubber gasket, the mentioned isolation washer does the same without the risk of causing corrosion to the aluminum. Cover the washer and spacer with mastic top and bottom, put in place and add stanchion base on top, tighten bolts lightly, so there is a thin layer (1/16" or so) of mastic left under the base, and between the washer and the aluminum. Tighten the bolts later when the mastic has cured.

Sikaflex 290 DC mastic including primer could be used. If you are concerned over UV-effects the beads could be covered with 295 UV, see Sika information
http://www.bluemoment.com/downloads/sikaflexmarinehandbook.pdf
Kind regards
Lars