Don't brush on topcoat. It is not designed for brushing or the underside of the car. That would create a horrible mess.

The grey primer with the POR-15 sounds like your best bet.

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2010 Toyota matrix

'93 500 SEL
A bad addiction. Takes all of my cash.

'12 Volvo S80 T6
Needed something that wasn't as hard to deal with as my bad addiction

'18 Mazda Miata
No more boring cars for everyday transport!