Sealing the hinge line is a very good idea to prevent flutter, but it won't help your loose linkage any.

First find out where the looseness is coming from. Is it from the long pushrod flexing too much? If so, you might need to stiffen them up a little. Wooden ones can be stiffen up very nicely by laying on carbon fibre tape with CA. Otherwise you may need to add midspan constraints to limit the amount of flexing.

If it is from loose fittings on the servo or control horns, you need to redo or reposition to get rid of the looseness. I have seen control surfaces flutter from loose linkages with the plane sitting on the ground at engine full throttle - without any aerodynamic load on it.

Definitely fix it.