Originally Posted by Dano57
...Vintage Trailer Supply sells two kinds 635 and 636 - what is the difference and which should I order.
Use the faster setting one (tack-free in 90 minutes, per the Vintage Trailer Supply web page) unless you know you need more than 30-45 minutes of working time. I found that it really wasn't tack free for something a little over two hours, but I believe the cure uses water vapor in the air (humidity) and I live in a low-humidity environment (Colorado).
Anyway, I was a little concerned about the working time, but it wasn't a problem. As a matter of fact, I'd like it to be even faster. The other type, the slow stuff, is really really slow, like still a little tacky the next day. Working time isn't that long, of course, but if you're wanting to get back to polishing, the slow stuff will stop you for several days. BTW, once curred, you can polish over it (polish, not compound or sand) and it completely resists the polish.
Hint on application. I put down a small bead, then fair it in with my fingertip. Then I take the corner of a plastic hotel key/credit card and use it to scrape the bead--removing extra Vulkem and getting a nice and tight bead against the seam (you don't need to do this on your belly pan bead, of course). Then I use a rag and mineral spirits to remove the thin film that has been spread by the plastic scraper, being careful to not remove too much of the bead in the process.