The 84 I'm doing has a thick layer of blue paint over a sanded down original medium green. Should I remove the blue or go to bare metal, and what is the best way to do this. I did one 25 years ago and used Zip-Strip on it. That was a mess but it worked.
If there is a lot of body work needed then it could be advantageous for you to strip it, like you said it is a lot of work and I mean a lot.
I'd sand it down close to the original finish but there is nothing like having a good sealed foundation for the new paint. I suppose that if there are large portions peeling off or flaking off then it is possible that you'll need to re-establish a bond to the metal surface.
So you have some considerations to think about now. I stripped down completely top to bottom over and under a 5th generation Elky and it was a chore. It took me weeks and I used chemicals, sanded, sand blasted and scrapped. I'll bet I removed over 300 Lbs of weight from the truck doing it. Paint in the 80s is not like the newer epoxy based paints, the finishes were not as robust and due to EPA the processes were changing so auto manufacturers struggled to get the processes down.
The body of my 80 was a piece of cake. The finish was letting go anyway, there was tiny little holes where voids existed in the original painting process. So when I started sanding I used 100 Grit and in just over a day it was all off the metal. Powder was everywhere. It was amazing that the finish held on as long as it did.
I'd only strip as a last resort because the potential problems to the new paint often don't show up until a year or two later and then it's a real problem, i.e.: spots peeling, blisters, etc. if you don't get every spot that the stripper touched completely neutralized and cleaned. I assume that the body is relatively "straight" without a lot of bondo, that you have done whateever rust or dent repairs that are required and that thick means thick, i.e.: some sort of enamel finish; accordingly, I suggest the following:
1. Make your first "cut" with 80 grit open coat paper on a DA sander for the curved surfaces and a 1/2 sheet electric oscillating pad sander or a long flat (3 x 12?) flat oscillating sander on the flat surfaces such as the hood, doors, tops of the quarters, etc. When I say first cut, I mean to just take the top off of the thick paint, but not to cut through it.
2. Do the same process with 180 or 220 grit open coat to where you just see the the original paint showing through, but do not go to the metal unless you have to.
3. Block sand wet with 220 or 300 grit wet or dry paper to the original paint, but again not to the metal unless you have to.
4. If you have a good original base from the original paint, you're ready to finish, if you're concerned about the original finish that's left, spray with the appropriate sealer and then the finish coats.
You'll have a job that is far better than original and if you "resort" to acrylic laquer as I like to do for the finish, you can do the whole job in your driveway. Of course you have to sand and buff laquer. It's a lot of elbow grease, but you will be able to be really proud of the finished product.