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I HATE undercoating. You have several options but truck bedliner lays on thick and stays better without being a tar that you can scrape off with your fingernail. Just buy the gun, mix with reducer and spray. Get a needle scaler to clean everything off fast.
However.. I was looking some more this morning (no reason why) and i came across POR15 - https://www.canadiantire.ca/en/pdp/por-15-gloss-946-ml-0475874p.html - and apparently this stuff is like way good.
Undercoating never fully dries and is always like tar except for where it flakes off. The bed coating materials are more expensive but are way more durable and they harden up nicely. Back in the day (43 years ago) undercoating was the only option. I just filled my garbage can with all of the undercoating and crud I took off of the Blazer. It served its purpose but I resprayed everything with bed coating for a rust proofing and it came out so nice. Here's a pic of the gas tank. It will not scrape off under your fingernail and will last for years.