Definitely paint the floor and use a seam sealer where there may be less when applied at factory or where you have had any repairs. It comes in caulk style tubes and cans. It never really dries hard but does to touch. Then yes apply paint. Spray brush depends on type you get or have and what set up you use...No one see's it under the insulation and carpet. Many here will have preferences on the paint type or the name brand. I say paint because enamel will work to seal the metal from moisture as long as there is no rust you are covering. If there is you need to remove it or deactivate it using something like Royal jelly or Rust Mort or the like to slow or stop the rust from creeping through the area. Best is to remove it.
Not sure what brands people will say that is sure to come...but treat repair the rust and seal with a paint...many say use an epoxy type to seal but again if you have rust under that it won't stop it from spreading.
Then ask yourself do you want a newer insulation on top of the paint like a Dyno mat product or the original style insulation with layers of materials that soak up moisture and hold it there...That is up to you and your budget

 

There def will be a product like dyno mat after painting....personally, I was just gonna roll some rustoleum on it....I guess I'll wait to see others suggestions.
Thanks!

 

Bed liner may also be an option? it's a little thicker, goes on with a roller and typically has rubber bits in it to help dampen things.

 

Hmmmmmm

 

This is a lot easier than POR-15: Heavy-Duty Anti-Rust in Black - Eastwood

 

Make sure that sound deadener would stick to the surface left behind by bedliner. I would use epoxy primer instead, you can spray or brush/roll it.

 

I painted my floors inside and underneath with the Master Series line since I could brush it on. Top coat was their
Mastercoat Chassis Black (AG111).

Still looks fantastic 15 years later.

Mastercoat Chassis Black Ultimate Frame Finish Paint

The Master Series Coating Line & Mastercoat's gloss black Chassis coating finish coat. Durable and chemical resistant. Low prices.

nomorerust.com

 

Check out second skin for their options. I use 2 if their products. But since you are able to spray the floor they have an option that works for you.

 

I used Lizard Skin, then Cool Mat from Summit. Reduces noise, heat. Lizard Skin sprays on (use both kinds} very little overspray, you can mask first 12 inches of surrounding stuff and have no mess.

 

Lizard Skin for me and then a coat of catalyzed 2K clear over the top of it to give it a very smooth surface. Then I laid down one of the heavy adhesive-backed rubber products on top of that smooth suface and it has all stuck like glue. Don't forget to do the ceiling for an overall much quieter vehicle.

Rick