Waterproofing electronics by dipping into polyurethane or lacquer
To expand on a brief comment: do not dip a board into any woodworking polyurethane as a conformal coat, unless you really like gambling.

First, you have no idea whether or not the solvents in the varnish are compatible with your parts.

Second, dipping into standard polyurethane will produce a thick coat, and drying stresses may damage your board. If you must do this, try a spray formulation which will give the thinnest coat possible.

Third, no matter what conformal coat you decide to try, the board must be scrupulously clean. That means cleaning with alcohol, and handling with cotton or latex gloves afterward. Adhesion to PCBs can be tricky.