Removable/soluble potting compound?
The ability of a material to mechanically protect components, keep water out, and withstand environmental conditions is in direct opposition to being able to easily remove it. Ultimately every compound is soluble in something (water, alcohol, acetone, benzene, piranha bath, aqua regia, HF...), but the problem is that components are too. 

Depending on your application, you may consider a thin conformal coating, potting with a low melting point compound such as paraffin wax, or placing the circuit in an environmentally sealed enclosure. You may also redesign the circuit itself to have self-test capability, bring test points out to a connector, fail less often, or be cheap enough to throw away when it breaks.