Teardrops n Tiny Travel Trailers

Hi,
Question about picking the right part of the trailer that are wood to waterproof. Maybe I am struggling because I don't understand the principles....
I just bought a prebuild flat bed (see picture). My plan is to create an inner frame and sandwich glue the inner frame with my 2" insulation wall against the outer frame of the flat bed(I saw that idea in one of the bug out foamie videos). I just picked up spar polyurethane and paint thinner and will use the mix on them.

1) On the Bottom should I
- a) Flip my trailer upside down and paint the 2x4 woods that stick out? -
- b) Or should I remove the plywood and paint all of the 2x4 and the covered (bottom) of the flatbed's plywood floor and then screw the floor back? A lot More work but I will do it if it's worth it.
2) Should I paint The exposed (top) of the plywood floor of the flat bed? (That I will lay my foam board on) - If I do 1a, then I probably want to do this, right? If I already do 1b, then will skip this step or should I do it too? Also, This one I have to make sure I don't paint the part that I will glue the foam on, right?
3) The 2x4 outer frame of my flatbed's floor? - This one I will cover it with PMF 's canvas and glue on later so I should not waterproof them now, right?

I think I only need to do 1a and 2 and have a good coverage there. What do you think?

I am not familiar with your trailer's construction but generally treat all exposed surfaces. Should not need any disassembly nor should surfaces to be covered with pmf require treatment.

I could be missing something as I've never seen a trailer like or even very similar to the one in the photo.

Thank you for your help, QueticoBill!

Kim,
I think you should do 1a. plus the bottom of the plywood as well. The mix is 50/50 equal parts of Urethane and thinner. This mix will soak in and require at least 3 to 4 coats to get it a good waterproof covering on the wood.
Leave the top of the plywood bare, and you could rough it up a bit (sand paper) before you glue the foam to it. I used construction adhesive called (tough as nails) to glue my foam board down to the plywood. Comes in a large caulking tube. Squirted some out and used an 1/8" notched trowel to spread evenly across the floor. Then install foam and weight it down with anything heavy that you have...lol. let it set 24hrs and it will never come off again.
Hope this helps.

Thank you for your advice YJlover63, I could not find the adhesive you mentioned, @home depot, there is something similar "Liquid Nails Extreme Heavy Duty 10 oz. White Interior and Exterior Construction Adhesive" $3.50 for 10 oz, does that sound right?
Do you spike the foam before glueing as well?
There is Gorilla Heavy Duty Construction Adhesive, but it's twice as expensive, price is not a problem since this seems so crucial.
Which one should I choose? and why did you go with this option?

I just found this video on youtube saying that Gorrila glue or Loctite is better...
https://www.youtube.com/watch?v=uBWetxzqnIs

PL Premium 10 fl. oz. Polyurethane Construction Adhesive is at $5 per 10oz
Gorilla construction glue is at $10 per 9oz

Seems all products are safe to be used on foam boards and wood.

Generally an acrylic or latex glue us better with foam, and cleans up with soap and water, but nearly any adhesive will work glueing foam atop wood. The loctite seems a little lower cost.

The PL premium is the same as what I used just made by a different company. I seen on here all kinds of reviews about the GG or Gorilla Glue, but I personally never had any luck with that. I tried it a few times, it worked good for me wood-to-wood if you wet the surfaces 1st. But not the foam to wood. Maybe it was just me.

I didn't score the foam in any way, just stuck it down and added the weight on top of it. Let it set for 24 hours and then put the glue on the top side of the foam and set the floor on it. Added weight. let sit, done. My top plywood (actual floor) is only 1/8" and I walked on it with no issues while finishing the build. I am 6 ft tall and 230 lbs.

The foam that I used was the 2" thick 4'x8' sheets of pink foam board.

Kim-
My curiosity is getting the best of me. Is your trailer basically wood framed? I don't see any steel framing. No qualms or reservations, just not quite like any I've seen.

Thanks.