Geocell makes some great products. We use their 2300 to hand seal loose shingles or where we have to make a tie into an existing shingle roof. Silicone is NOT a good sealant when used on asphalt shingles. Geocel makes a excellent brush on grade product that we have used to repair the mortar wash (mortar cap) on top of true masonry chimneys.
I don't like their gutter sealant because we are usually trying to repair a leaky seam in a wet environment and it has a tendency to ball up. I think it would work good on a dry application. There are wet and dry gutter sealants available for sealing existing seams or mitres, cleaning the joints is very important on existing repairs.
If you would at least primer the back of your fascia boards you will be light years ahead of what is installed on most houses. Primmer minimum. I like to over kill cause when I'm off work I don't like to do repairs on a house. Kinda like the shoe cobblers kids running around barefoot.