How can I minimize the moisture adsorption on FR4 PCB?

Adsorption is a chemical process that occurs on the surface and can affect surface resistivity. Since you are talking about PCB mass, it sounds like you are actually talking about moisture absorption which is diffusion into the bulk of the material.

If there are any fixing holes, it will be impossible to eliminate them completely.
I have some products where the use of this sealant is mandated (they are in ship-board aircraft). I only mention this to show there are solutions, if there is a requirement for complete sealing.
Simply covering most of the PCB with solder mask will reduce the absorption of moisture.
From a system point of view, other than the bare PCB, it would be important to avoid components that absorb moisture- which would include plastic-packaged parts, but especially polyamide (Nylon) fasteners etc. which can absorb an enormous amount of moisture (as much as 10%)- enough to cause large dimensional changes as well as mass changes.

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Oliver Smith

Oliver is an experienced electronics engineer skilled in PCB design, analog circuits, embedded systems, and prototyping. His deep knowledge spans schematic capture, firmware coding, simulation, layout, testing, and troubleshooting. Oliver excels at taking projects from concept to mass production using his electrical design talents and mechanical aptitude.