I keep seeing all these custom body modifications that look really nice on the cars when they are finished, but I wonder what the long term effects are? Example......Frenched in bumpers. All the slicing and dicing with seams welded up. Won't it show a shadow "ghost" line effect eventually? I am curious because I have thought about similar modifications myself. But I don't want to be re-doing everything 5-10 years from now. Trust me this car will never leave me........it's an heirloom for my son. It took me 15 years after my first 68 to get another one. I have seen an example of some fine craftmanship on Detoit Speed's web-site. It's on Michael Manning's 69 Camaro. Looks awesome! just wondering if properly done, will it hold up to the test of time? Way back in the 80's my dad had a 38 chevy coupe that was one of Rod Action magazines top 6 cars of the year. Excellent craftsmanship but things were starting to "show" after a couple of years. That car was top notch. I like the modifications, I was just thinking in time as original as possible might be the best way to go?

Most Common problem is due to vibration near a welded area. Second most common problem is the lack of weld thru primer. Another problem is the lack of penetration on welded areas. If you were to chop & seam a area say a bumper if you do it all top scale it will last for quite a while if done right, if any step is short cutted you'll have a problem down the road. Best advice is to slow down and cover all the steps to make the process work the best with your given talent and tools.

Like Yenko above..If a repair, custom mod or anything EVER shows a shaddow or simlar defect it is due to the job not done right...
lets say a body chop...weld sticking willwork IF the welder knows what he is doing, BUT eventually it will show a line...If it is fully welded with the right heats, etc it takes a hell of a lot longer but will never show...IF the preparation over the weld is also done well.
Hence this is why some body shops can do work 'Cheaper' that a high end custom shop.
Its not materials ..its the time.
And not just the time to weld but to hammer file the seam rather than just just a heap of filler
Visit a craftman shop, and ask if u can just sit back and watch a guy at work..for a day.
It s a realy eye opener...not just how he .moves the metal. but the utter realisation of the patence of the guy.
It doesnt take a rocket scientist to do the work, just a good feel for the metal, commonsence, logic, eye for detail and perfectionist.
Just like say plastering a wall at home youself to paont, it has to be perfect or every defect will show as a shaddow...BUT unlike a good platerer trades man who picks up a trowel throws on the wall, then following day all it needs is a light sand..You will have to go over the wall several times, block sanding several times over more days to get the same result.

I notice when I am grinding down my plugwelds that on the edge of the heatzone I will get like a magnetic line where the metal dust will collect. Like they use on engines to find fine cracks right? Is this a sign that something needs to be rewelded?

I follow doing it properly, but it is to my understanding the idea of weld through primer is to have the area your welding painted with this primer before the welding takes place. That way you are sure to seal up all bare metal. If that is correct why is it at all these top notch shops they always do the work in bare steel? I never see weld through primer on before welding. Follow the pics. at Detroit speed or V8 T.V. Even Rad Rides appears to do it this way. ?

Well when you are doing a butt weld you can treat both sides after you are finished, so why would you use a weld through primer in a case like that?
Weld through primer is a pia, it really doesn't weld that well after it has been applied.