It is not something I have done myself so don't have a specific type, clear varnish is what others have tried. This may work on it's own.
Trying the laptop on battery would be a quick test to see if resolves the issue, not a long term logging solution. If it does prevent the jumps then the supply may inducing some noise, or that it is grounded, so a different supply may be better.
The other issue could be that the thermocouple wires are picking up noise from the circuit under test so consider rerouting the wires.
It is often a case of trying different solutions to work out which one is the correct option in your situation.