I'm currently building a trailer which will have a small fuel tank and some extras on it. I will post some picture when I get finished.
I wouldn't be so inclined to test with water. No matter what, there will still be some residual moisture.
To test with air, all you need is a simple psi gauge that will read 1psi increments. you only need maybe 5 psi at most, lock it off and let it sit and watch. If it holds short term, let it sit an hour and check again.
If you get any pressure loss, refill to 5 psi - get out the soap solution and start-a-sprayin'.
Much less expensive to discover this way than chasing drips......
I just made a post showing a fellow what I made last summer. I used the same Northern pump and water blocking filter..Total cost less than $200. I did have corrosion under the saddle straps from road salt and I cleaned that up prior to mounting it. I used an external fuel sealing compound to paint over the aluminum where I removed the corrosion. That isolates the steel bands from the aluminum and stops dissimilar corrosion. Easy to transport to off road fuel station.
TSC normally stocks the water/particle filter. I bought an extra when I got the base for my set up.Picked up a particle/water filter today with mounting base at TSC. I believe it is a CIM TEK brand. Only thing is I didnt see any replacement filters that was both water and particle besides the one that comes with the base. I only need one base, but this seems to be the only way they sell the dual purpose filter. They have plenty of water or particle, but one that does both of any brand, any where is impossible to find. Oh well this will work for now.