Seam sealer

One of the most important things to do when you're preparing your car for painting is sealing the seams. Most rust problems will start in seams if not sealed properly.
There're a lot of different types and brands of sealants on the market,
Never, never use a sealant with silicone in it for the simple reason you cannot paint over it.

I tried two brands; Sikaflex 227 and HB Body 115. I put a bead of both of them on an old sunroof and after letting it cure for a day I put it in the scorching sun for a couple of days. They both survived, but in my opinion the HB Body 115 has a better adhesion.

Sealants

Seams sealed in engine bay

Seam sealing bonnet

 Within 30 seconds after applying the sealant I dipped my finger in thinner and went over the sealant to smooth it. After all the seams were sealed I let the sealant dry for a couple of days and then brushed over it with epoxy primer.