Hope to have the 914 back running in the next few weeks but one of the things on my to do list is to figure out what to do with the fuel tank. I bought the car from my Son and its been his daily driver for the last 5 years. The fuel tank is somewhat rusty inside, I know that has been one of his headaches with the car was he was always having to change the fuel filter because it would plug up with rust. I have seen some sites that have coating for the inside of the fuel tanks but wanted to get some input from other 914 owners. Should I just change the fuel filter frequently or try to eliminate the problem.

Take the tank to a radiator shop and have it boiled. Then take pics of the inside and post here. Then you can evaluate your next step.

Fuel can cause fires when leaking. I am going to refurb my tank soon. I don't have the internal rust issues on the tank like you, but I have rust on the outside of the tank.

There are kits from Eastwood and POR-15 to get the internals of the tank resealed and if serious work needs done your tank can be taken to a radiator shop for repairs.

Or you can shell out $375+ shipping to AA for a refurbed one...

Radiator shop, get it boiled....
Or
A gallon of lacquer thinner, a three foot piece of chain....keep shaking and turning till you get all the crap out.

I have not heard anything great about liners except they will start pealing eventually.
We just boil them and fill them up with gas...and keep them full.