The easiest way to remove undercoating is to have someone else do the job!
Barring that, there is no simple easy way to do it. Some have used diesel fuel
to soften the undercoating and scraped it off, some have used CO2 to freeze it,
and chipped it off. But there is no simple and easy way to do it. Have a
caution with blasting, it's easy to lose metal that way.
For metal protection after it's been cleaned, you can use a panel prep wash.
That does a decent job of protecting bare steel. Better still would be to prime
and paint it. You can use a neutral color top coat to allow subsequent body
color matching if you wish after you finish the body.
I'd suggest you not use POR-15 here. It is not well suited for bare steel and
will have poor adhesion. Not that it has terribly good adhesion even on a good
day. It also must be protected, as it breaks down quickly when exposed to UV.
All in all, it isn't very impressive, and is not used here by anyone that wants
serious protection of metal parts. There are far better products.
If you really want to protect the frame, consider using a 2 part epoxy. Also
consider how well it is applied inside the frame, and consider stone damage.
Epoxy paints are strong, with good adhesion, but they do chip. An additional
coating of a flexing agent, like rubberized undercoating, will protect the epoxy
from impact chip damage. You can even use something like 3M StoneGuard and
paint over it for the body color match, and maximum protection.
For body inversion, the bonnet catches and bumpers are very strong points up
front. A 2x4 simulating the door is also a good inversion point. So too are be
the rear fender seams. At no time would I recommend you standing on the body
this way, but it should hold its own weight quite well this way. The many I've
seen flipped this way showed no signs of damage. Be very aware of the damage
potential while attempting the flip (this I have seen).
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