Q: When installing a new water pump and thermostat on my 1989 Pontiac grand Prix should I use gasket sealer or silicone grease

In your application, the new thermostat comes with a new rubber seal which is the entire gasketing solution for the thermostat housing. Nothing should be added as that may interfere with the seal. The water pump uses a paper/fiber gasket. So long as the mating surfaces on the pump and engine are perfectly smooth, have no pitting, are clean and down to bright aluminum, and are not warped, there is no need to coat the paper gasket with RTV sealant. If there are defects in the metal surfaces, sealant will help but if no defects are present the sealant is superfluous; the gasket makes the seal. A leak proof seal with a paper gasket in this application requires that a torque wrench be used to tighten the pump bolts in sequence to 89 inch pounds (7.4 ft. lbs). YourMechanic can dispatch a certified mechanic to troubleshoot this if there is a leak. The service to request would be pump installation. Please let us know if you have further concerns or questions.