This is why I love this forum, I love the knowledge here.
I decided to go with that gun I posted, I found that a very similar gun was sold at NAPA for a while for the same price range, and people had good results with it, fingers crossed.
I also decided after much debate, to go with fluid film, I'm doing all this within a budget, and considering the fact that both vehicles I will be undercoating have rust. I have found anecdotal evidence that fluid film penetrates rust and at least slows it from spreading, no doubt carwell does the same, but FF seems to have more of a resilient coating that I think could benefit a vehicle with rust better.
I was considering making a video, showing the before, and after of both vehicles, seems mostly pointless other than to show the shine and gloss a good undercoat may give, I'm not sure, if I do, I may make a new topic incase someone may be interested in the look.
I'm honest to god excited to do this, hopefully this little 10 gal Central Pneumatic air compressor is up to the task