What we have here may look like a mess, but it is actually a collection of goodies that will keep the stainless from touching the aluminum. Teflon tape, also known as plumber's tape, is laid out over the entire back of the block. This will provide a base layer of separation between the two components.
The next layer of separation is polysulfide that is piled into the middle of the block. This overfilled amount of material will ooze out over all the surfaces and provide an additional layer of separation and keep any water from getting in between the block and the spar. Polysulfide is great because the excess simply oozes out and is then allowed to cure. Once cured, it can be cut off for a clean, professional looking installation.
The brown stuff in the corners is Lanocote. This stuff should really be called "wonder grease" because it does everything you could ever dream of, and it's non-toxic. Lanocote is just wool nap, the grease that sheep produce to keep their wool waterproof. This stuff is probably the only material that you can smear on with your finger and not worry about getting cancer from it in 20 years! It acts as a separating layer, anti-corrosive, and anti-galling. Best of all, Lanocote is very inexpensive!
I put Lanocote in the corners since these are the only parts that are actually going to touch the spar. The rest of the plate is merely "close" but not in actual contact. The Lanocote also helps hold the teflon tape in place while the block is being positioned, that way it doesn't slip out of the way when you are not able to see it.
All of these layers might sound wonderful, but there is still one serious fatal flaw in this setup. The screws themselves are stainless steel and will be in the most intimate of contacts with the spar. They have to be holding onto the aluminum or the block will fall off!
The secret here is once again, Lanocote. Dipping the screws in the pot of Lanocote will pick up a lot of material that will coat all the surfaces of the screw. The Lanocote will get into all the nooks and crannies between the threads and down the shank, everywhere that the aluminum will also be in contact.
When you drive the screw into the spar, the Lanocote will coat everything and provide all the necessary qualities needed to isolate the two metals while still allowing them to grab onto each other. This will prevent galvanic corrosion and keep all the metals happy, granting you a long service life with happy components.
You might think that this must be too good to be true. How can such a wonderful material be inexpensive and easy to use?! Unlike other more expensive isolation creams and gels that can be purchased, Lanocote will stand up to the test of time. I have removed old components that were attached with synthetic isolators, such as Loctite, and found signs of galvanic corrosion beginning. On that same note, I have removed components that were installed 20 years ago and there were no signs of corrosion between the aluminum spar and stainless steel fasteners. The Lanocote was still moist, just like the day it was applied!
If you ever need to attach two dissimilar metals, be sure to apply a liberal coating of Lanocote between them to extend their service life and give you peace of mind that all your boats metals are happy.