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If you buy or borrow a moisture meter to use while looking over a boat for possible purchase there are a couple of things to keep in mind:
1. The meter measures the radio frequency electrical conductivity of the material it is in contact with (and below the surface to a variable depth). The meter does not directly measure water. Many materials, including fiberglass laminate and various gelcoats and paints, have some RF conductivity. You will not get a zero reading even on "bone dry" fiberglass. It will vary to some degree with the type of resin and fiber (typicall about 10%-15% on the "wood" scale see #2 below). Of course, water saturated materials do have higher RF conductivity. Primarily what I look for is large variations around chainplates, deck mounted Genoa track (often a culprit), and other penetrations.
2. The "Percent" scale is for WOOD only - not fiberglass. On fiberglass you can only use it for relative measurements. Look for anomalous areas - real wet deck core usually will "peg" the meter.
3. If the boat is wet stored, or has only recently been hauled, the meter will not tell you very much about the bottom below the waterline. You probably need several days and perhaps more, depending on how long the boat has been in the water. All resins (including epoxy and vinylester) have a finite diffusion coefficient for water (i.e. they all will absorb some molecular water over a long enough period of time). In New England, where boats typically spend less than 6 mo/yr in the water, the bottoms should dry quickly. In Florida and TX where they sit in hot water for years at a stretch, resins will absorb quite a bit of water over that time. On a 1 hr quick-haul for survey (normal here in TX) the bottoms ALWAYS read wet on a meter.
Hope this helps you-all.