Modern Vespa

I have a 2018 300 bought used about 3 months ago. Appears to have been garage kept and very low miles. I decided to check on the battery as I am too old to push. I was pleased to see a plastic tray, clear drain and minimal terminal crud. The pan has some very preliminary rust and I want to catch it now before it progresses. I have ordered ACF 50 spray but it comes by mule train and I am thinking of 3m undercoating for now. Any thoughts? I live in the southern US , ride in warm dry weather. BTW the ORIGINAL Yuasa battery is still resting at 12.7 volts. Wish they made auto batteries.

Link below might help, also make sure that battery has no leaks that could have caused that rusting in the first place.

Rust in battery compartment

I gave it a cursory clean and sprayed the well with rubberized undercoating.
Several coats and time to dry. The gel battery that appears to be the original has no vent / tube. I can understand how owners in salted climates battle this problem. The topside of the floorboard is difficult to access. It appears that water will infiltrate and pond despite the drains. While shopping I saw one 3 year old model that must have been ridden in the surf( Florida). The spine was rusted away. Since this prevents the scooter from folding like a jackknife, it had no takers. I feel sorry for any vehicle that operates on salted roads. It seems like a when/ not if problem. The vehicles in my area stay bright and rust free. It is disheartening to see videos of stuff from the UK.
I have two sedans, one 10 and the other 20 YO. No rust visible anywhere.

Rust on cars was dramatic over here in the sixties and the seventies, but has been solved since the early 80's by using galvanized steel plate.
One of our cars has a scar running over both right doors. It is a deep scar disforming the plate. It goes way through all of the paint.
The incident happened in 2019 and today, almost 6 years later, still no trace of any rust.

Would Rhino lining be an option after cleaning it up?

Maybe others have more knowledge.
The plastic floorboard covers the actual steel tray. I managed to seal the battery well but trying to spray sealant into the small void was too difficult. I have a pickup bed with a reputable sealer. It is 20 YO and except for gouges it is intact. Holds water still. I do not want to be an alarmist but these scooters are relatively expensive. People who ride them daily must have better ideas. I can simply choose to ride another day.
I will caution that the Zenier? Diode lives in that area and will not thrive with an undercoat.
On another topic, I replaced coolant upon purchase. CEL came on two weeks later. I added coolant- light goes out. Today after 20 miles at highway speeds, comes on again. Reservoir almost full but added a little. Still light on. I opened bleed valve and got liquid but no air. Light goes off. Checked the engine oil temp. 200 degrees. No hot light- no fan. I don't believe in self healing. Any ideas?