Zinsser Gardz

I just bought a can of Zinsser Gardz for an interior project (bathroom) and am pretty impressed with it. It is designed to seal and prime damaged drywall so it can be patched and painted. There is nothing on the label to suggest that it can be used for exterior applications or for wood.
So now, I'm looking for a similar water-based product for exterior wood. I want to use it to seal hard-to-reach places in exterior woodwork such as the spaces between tightly-spaced-together porch floor boards, and the area where railing posts are sitting on the porch floor boards, to seal out water.
What I want is:
- water -based
- thin consistency so it will wick and soak into tight spaces
- forms waterproof seal
- dries clear and is paintable
- matte or satin finish (definitely not gloss)
Anybody know of a product like this?
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