Slopoke,
You are right and wrong.
Mag mounts at meant to be temp and changed from vehicle to vehicle or removed when not in use. you can not maintain a consistant swr with a mag mount if you keep moving it around. It is the relationship with a vehicle and it's location on the vehicle that dictates the swr.
any swr below 2:1 is considered good, 1.5:1 is better and 1:1 is perfect.
You can protect the sheet metal by using a spot primer and some touch up paint once you drill your holes to cover up any exposed metal. then you tune your antenna. by mounting it permanently you can tune it and almost forget it (should check it every now and then)
I promise you that your swr on your mag mount has changed IF you have moved your antenna. Not to mention that when u use a mag mount you trap dirt between the mag and the paint and screw up the paint anyway.
As long as the antenna is above the highest point of the vehicle you should be able to achieve a 2:1 swr. I just did with a door jamb mount using my rear tail light screws on the pass side. had to drill one extra hole but soild as a rock and a 4 foot antenna and i got a 1.6:1 on both channel 1 and 40 with 1.5:1 on channel 20.

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