funny you should ask... I just got done doing some seam sealing on my 57 around the new wheel tubs. I like 3M "Fast n' Firm" # 08505. I have used it on several projects with nothing bad to say about it.
I used one of the 3-M ones. It was the brushable one (thick as hell, but it was actually brushable). Painted right over it, no problems. I only have put DPLF 50 (50 is the grey, 90 is the black, right?) over it. Been a little over two years painted, havent gotten around to shiney paint yet.
I painted over some "fast n firm" 5 years ago and now the paint has cracked in quite a few places. I would be careful where I used it, if I use it again. Red
SEM makes agood product. A little pricey and you need a special gun (like a caulking gun) to apply it. It won't crack and can be sanded and painted.
If you want a seam sealer that won't crack, you have to use a 2 part seamsealer. 3M, SEM, Fusor, are just a few companies that make it. It comes in two tubes (attached to each other) that mix in a special tip while you are applying it, you need a special gun to apply it, your local jobber should lend you one, if you buy their product. paint can potentially crack when you put it over any one part seamsealer because the one part seamsealer expands and contracts more than the paint will, there are lots of situations where paint hasn't cracked over one part sealer, there are also lots of situations where it has, if you want to use one part sealer, put flex additive in BOTH your primer and paint, this will help prevent cracking. Also a tip while sealing, put some wax and grease remover (cleaning solvent) in a squirt bottle, when you apply the sealer, before you smooth it out with your finger, or brush squirt some wax and grease on it so it's wet, it makes the sealer waaay nicer to work with.
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