Hi - If you are talking about the transmission crossmember that the gearbox mounts on, then as this is made of steel I would make sure that a good coating of either zinc or lead based primer is applied first. I have heard good reports of POR15 as advised above, but never used it - but it does appear to be recommended many times on this forum
Once it is all primed up, I would recommend that you use a hard-wearing paint such as an enamel or synthetic. There are many paints that are developed especially for chassis components, so you should be able to source something suitable quite easily.
However, you can use practically anything you like really, but if you want the finish to last (as it's a harsh environment under there) spend a few extra bucks and get a tough paint. That said, Porsche just used basic black most of the time...!
High-temp paint is unnecessary, underseals are generally a bit sticky and on a component they never leave a good finish (IMHO) - great for floors, chassis rails, etc...but, as always, it depends on what finish you want to end up with
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